The process of baking meringue is a cross between baking it and drying it. Preheat the oven to 225 degrees F.
Prepare the meringue cake layer: Place the egg whites in a large mixing bowl and whip with an electric mixer on medium speed until foamy. Make a French meringue by adding the granulated sugar 1 tablespoon at a time. Increase the mixer speed to medium-high and whip to stiff but not dry peaks, about 7 minutes. Delicately fold in the powdered sugar with a rubber spatula. It is important to fold as gently as possible to avoid deflating the meringue.
Place the meringue in a pastry bag fitted with a 3/4-inch plain tip. Draw 2 (8-inch) circles on a sheet of parchment paper by tracing around the outside of an 8-inch cake pan with a pen. Turn the paper over so the drawing is face down (so that the ink does not bake into the meringue) and place on a baking sheet.
To pipe the disks, hold the pastry bag at a slight angle about 1 1/2 inches above the parchment paper. This height and angle allows the piped meringue to hold the full shape of the 3/4-inch tip. Start at the center of the circle and pipe the meringue in a spiral to the edge of the outline. (I work counter-clockwise because I am right-handed.) If the meringue breaks while you are piping it, just continue where it broke. If any air bubbles appear on the disk, go back and fill them in with the extra meringue left in the piping bag. It takes practice to get the disks perfectly round. Don't worry if yours are slightly misshapen. You can gently scrape away the imperfections after the disks are baked.
At this time, pipe additional swirls that are about 2-inches wide and the height of your cake mold. These will be used to decorate the outside of the cake.
Bake the meringue disks until they are firm to the touch, about 1 hour. If the meringue begins to take on color while baking, lower the oven temperature by 50 degrees F (10 C). Remove the baking sheet from the oven and place on a wire rack until the meringue disks have completely cooled. If you have the time, turn off the oven and cool the meringue on the baking sheet in the oven. This will take about 1 hour and will allow the meringue to dry completely.
While the meringue disks are baking, prepare the ice cream.
Prepare the ice cream and allow enough time for them to harden in the freezer.
Have the ice bath ready before you begin. To prepare an ice bath, fill a 4-quart bowl with ice cubes. Generously sprinkle salt over the ice and add water to cover the ice. Place a clean, dry 2-quart bowl over the ice bath.
To prepare the chocolate ice cream base: Heat the milk and chopped chocolate together in a 2-quart heavy-bottomed saucepan (this will keep the milk from burning) over medium-high heat until bubbles form around the edge of the pan. While the milk is heating, combine the sugar and egg yolks and whisk until thoroughly incorporated and thick. Temper the egg mixture by pouring about half of the hot milk into it and whisk well. Now pour the tempered egg mixture into the remaining hot milk and chocolate. Cook over medium-high heat until slightly thickened, stirring constantly with a rubber spatula. The mixture has finished cooking when it is thick enough to coat the back of the spatula, about 3 to 5 minutes. Use the following method to tell if it is finished: Quickly dip the spatula into the hot mixture and hold it horizontally in front of you. With the tip of your finger, wipe a clean line down the center of the spatula. If the mixture holds and does not fill in the line, it is ready to be removed from the heat. If the line fills with liquid, the mixture must be cooked a little longer. Repeat the test every 30 seconds until ready. At no time should the mixture begin to boil. If you see bubbles begin to form around the edge of the pan, remove the pan from the heat and whisk the mixture thoroughly to cool it down. Once the mixture has finished cooking, pour it into a clean bowl and set aside to cool. If you want to reduce the cooling time, you can cool the chocolate ice cream base over an ice bath. When the mixture is cool, spin it according to the type of ice cream machine you are using. The ice cream is ready when it is creamy and smooth, like the consistency of soft-serve ice cream. Place the ice cream in an airtight container and store in the freezer until ready to use.
While the chocolate ice cream base is cooling, prepare the vanilla ice cream base. The technique is the same as above; heat the milk, cream, honey and vanilla bean in a nonreactive 2-quart heavy-bottomed saucepan over medium-high heat until bubbles form around the edge. Before adding the vanilla bean, use a sharp paring knife to slice the vanilla bean in half lengthwise. Separate the seeds from the skin by scraping the blade of the knife along the inside of the bean. Add the seeds and the skin to the mixture. While the milk is heating, combine the sugar and egg yolks and whisk until thoroughly incorporated and thick. Temper the egg mixture, then pour the tempered egg mixture into the hot milk and cook over medium-high heat, stirring constantly, until slightly thickened enough to coat the back of the spatula.
Remove the pan from the heat and pour the mixture into the bowl placed over the ice bath. Remove the vanilla bean. Stir the mixture occasionally to distribute the cooler parts. When completely cooled, remove the bowl from the ice bath and puree the mixture until smooth with an immersion blender, food processor, or blender. Spin the ice cream in your ice cream machine and store in an airtight container in the freezer until ready to use.
The next step is to mold the ice cream into the cake pan. Line a cake pan with parchment. You may need to slightly soften the ice cream. Make a 1 1/2?inch thick layer of the chocolate ice cream in the bottom of the cake pan. Make a 1 1/2?inch thick layer of the vanilla ice cream on top of the chocolate. Place the cake pan in the freezer and let harden. If you want to decorate the ice sculpture with ice cream balls, use an ice cream scoop to shape the remaining ice cream into ice cream balls of various sizes. Place the scoops on a parchment paper lined baking sheet and place in the freezer until hardened.
Once the meringue disks are baked and the ice cream is molded, it is time to assemble the Vacherin. Remove the cake pan from the freezer. It may be necessary to dip the pan in warm water for a few seconds to unmold the ice cream. Once unmolded, place a meringue disk on top of the molded chocolate ice cream. Cover with a plate or cardboard cake circle and flip so the vanilla is now on top. Place a meringue disk on top of the vanilla and place the molded cake back in the freezer.
To make the whipped cream: Use a sharp knife to slice the vanilla bean in half lengthwise. Separate the seeds from the outside skin by scraping the blade of the knife along the inside of the bean. Place the heavy cream, sugar, and vanilla seeds in a medium-size bowl and whip to stiff peaks with an electric mixer on medium-high speed.
Remove the Vacherin from the freezer. Fill a pastry bag fitted with a 3/4-inch star tip with the whipped cream and use it to decorate the top and side. Using a turntable will make this task easier. Keep the Vacherin in the freezer until ready to serve.
It is easiest to cut the cake with a hot serrated knife. To heat the knife, dip it in a tall container filled with hot water before each cut and wipe off the excess water.