Line a 3-quart metal mixing bowl with plastic wrap, and leave the edges overhanging. Place lined bowl in freezer for 10 minutes; this will be the mold. Chill the bowl of an electric mixer as well. Place the pistachio ice cream in the chilled mixer bowl. Using the paddle attachment, beat on low speed until ice cream is spreadable, about 30 seconds. Remove chilled mold from freezer. Using a rubber spatula, coat inside of mold with all the pistachio ice cream, making a 1/2-inch-thick layer. Return mold and cleaned, empty mixer bowl to the freezer for 30 minutes. Place the vanilla ice cream in chilled mixer bowl. Beat on low speed until spreadable, about 30 seconds. Remove mold from freezer. Using the rubber spatula, coat pistachio layer with the vanilla ice cream, making a 1/4-inch-thick layer. Return mold and cleaned, empty mixer bowl to freezer for 30 minutes. Place sorbet in chilled mixer bowl. Beat on low speed until sorbet has softened, about 1 minute. Fold in wafer pieces. Remove mold from freezer. Pack sorbet into center, making an even layer 1/3-inch below ice cream. Cover sorbet portion with sponge cake; trim cake if necessary. Return mold to freezer until completely hardened. To serve, place a serving plate upside down on top of the mold. Holding plate against mold, invert both. Place a hot, wet kitchen towel over the mold. Remove the metal bowl. Immediately before serving, remove plastic wrap.
Place the sugar and 3/4 cup water in a small saucepan. Bring to a boil over medium heat, stirring occasionally. When sugar has dissolved, remove pan from heat, and stir in the creme de menthe, if using. Chill the syrup. Remove the seeds from the watermelon flesh. Place the flesh in the bowl of a food processor; process until liquefied, about 2 minutes. Add puree to chilled syrup; stir to combine. Freeze sorbet in an ice-cream maker, following manufacturer's instructions. Store, frozen, in an airtight container. Yield: 1 quart
Heat oven to 350 degrees F. Butter 2 (6-inch) round layer-cake pans. Line pans with parchment paper, and butter again. Flour the pans, and set aside. In a small bowl, sift together the flour and cornstarch; set aside. In the bowl of an electric mixer fitted with the whisk attachment, beat the egg yolks, vanilla, and 4 tablespoons sugar on high speed until thick and pale, about 5 minutes. Transfer the egg-yolk mixture to a large bowl. Wash and dry the mixer bowl and the whisk attachment. Combine the egg whites and salt in the mixer bowl, and beat on medium speed, until whites hold soft peaks, about 1 1/2 minutes. With mixer running, slowly add the remaining 2 tablespoons sugar. Continue beating until stiff and glossy, about 1 minute. Fold the egg-white mixture into the egg-yolk mixture. In three additions, fold the reserved flour mixture into the new mixture. Transfer the batter to the pans, and smooth the top with an offset spatula. Bake until a cake tester inserted into middle comes out clean, 35 minutes. Transfer pans to a wire rack to cool; turn out the cakes, and wrap them in plastic wrap until ready to use. Yield: 2 (6-inch) round layer cakes