a 17-by-11-inch silicone baking mat; a cake comb with 1/4-inch spaced teeth; a 17-by-11-inch jelly roll pan; four 3-inch diameter pastry rings; a piping bag; a piping bag fitted with a large open star tip (4B); kitchen-dedicated tweezers; a small disposable piping bag; a small star candy mold with cavities approximately 1.1-inch in diameter
For the red stencil paste: In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and confectioners' sugar together until the sugar is dissolved. Add the egg whites and mix again until incorporated. Stir in the flour and red food color; mix until just combined. Scrape down the bowl and mix again briefly.
With an offset spatula, spread a thin layer of stencil paste over a 17-by-11-inch silicone baking mat. Using a cake comb with 1/4-inch spaced teeth, and starting at one corner of the mat, drag the comb diagonally across the mat and through the paste so that 1/4-inch even stripes of red paste remain on the mat. Repeat the combing process until the entire mat is striped. Place the mat inside a 17-by-11-inch jelly roll pan and freeze for at least 10 minutes. (You may have leftover stencil paste.)
For the sponge cake: Preheat the oven to 450 degrees F.
Using a stand mixer fitted with the paddle attachment, beat the almond flour, confectioners’ sugar, all-purpose flour, whole eggs and egg yolk until combined. Stop to scrape down the sides of the bowl as necessary.
In a separate bowl, whip the egg whites and granulated sugar with an electric hand mixer until a thick, glossy meringue forms. Gently fold the meringue into the almond mixture. Add the melted butter and gently fold again, being careful not to deflate the batter. Remove the jelly roll pan from the freezer and pour the batter on top of the red stencil paste. Bake for 5 to 7 minutes, or until the cake springs back when pressed in the center. Remove from the oven and let cool slightly. Run a knife between the cake edge and the pan to loosen the sponge.
Invert the cake onto a sheet of parchment paper and carefully peel away the silicone liner. Trim the edges of the cake away using a small sharp knife.
Place four 3-inch diameter pastry rings on a baking sheet. Cut the striped sponge into 3-inch-wide strips and trim the length to fit inside the pastry rings. Line each ring with the sponge so that the red stripe design rests against the pastry ring. Refrigerate the cakes while you prepare the peppermint mousse.
For the peppermint mousse: Place the 2 tablespoons cold water in a condiment cup and sprinkle the gelatin over the water. Let stand for 1 minute. Place the bloomed gelatin in a medium saucepan. Whisk in the egg yolks and granulated sugar; mix well. Stir in the hot milk. Cook over medium heat, whisking constantly, until thickened, 7 to 10 minutes. When done, it should be thick enough to coat the back of a spoon. Stir in the chopped white chocolate and let stand in the hot mixture for 2 minutes. Add the peppermint and blend with a whisk until the chocolate has melted and is even in color. Let cool to room temperature. Meanwhile, whip the heavy cream to soft peaks with an electric mixer.
When the chocolate mixture has cooled, pour it into the whipped cream and add a few drops of red food color. Gently fold the whipped cream into the cooled chocolate mixture until well blended. Transfer the mousse to a piping bag.
Remove the cakes from the refrigerator and place each cake on a length of plastic wrap double the size of the pastry rings. Pipe peppermint mousse into the centers, filling them to the top. Cover the cakes with the overhanging plastic wrap. Refrigerate until set, about 2 hours.
For the Christmas tree toppers: Combine the butter and confectioners’ sugar in the bowl of an electric mixer fitted with the whip attachment. Beat until combined; add the milk or cream a little at a time until the mixture comes to piping consistency. Mix in the vanilla extract. Set aside 2/3 cup of the frosting in a small bowl and cover with a damp paper towel.
Add the green food color to the remaining frosting and beat well to combine. Transfer the frosting to a piping bag fitted with a large open star tip (4B).
When the cakes are well chilled, remove them from the refrigerator and cover the tops with a thin layer of frosting from the bowl of reserved white (untinted) buttercream. Place a sugar cone in the center of each cake. Pipe green buttercream stars on the cones beginning at the bottom edge of each cone, and travelling upward to the pointed end. Immediately sprinkle the cakes with the white nonpareils. Place gold and white sugar pearls on the trees using a pair of kitchen-dedicated tweezers. Arrange the small candy cane sprinkles on the tree using the tweezers.
Melt the yellow candy wafers in a microwave-safe bowl in the microwave at 100% power in 30-second increments. Stir between heating until the candy is smooth. Transfer the melted candy to a small disposable piping bag. Pipe the candy into a small star candy mold with cavities approximately 1.1-inch in diameter, filling just 8 of the cavities. Place in the freezer for 5 minutes, or until hardened. Turn out the stars onto a baking sheet. Use leftover melted candy to attach the flat sides of two stars together, lining them up to match evenly. Let stand until firm, about 2 minutes. Place an assembled star on the top of each tree, pressing them into the icing to adhere firmly. Refrigerate the cakes until ready to serve.