Special equipment: Four 8-inch cake pans, one 9-inch (2-quart) tempered glass mixing bowl and two 8-inch cardboard cake rounds
Preheat the oven to 350 degrees F. Grease and lightly flour four 8-inch cake pans and one 9-inch tempered glass mixing bowl.
Prepare the vanilla cupcake or cake mix batter. Mix the crushed sandwich cookies into the batter.
Fill a 9-inch tempered glass mixing bowl halfway with batter. Divide the remaining batter among the cake pans, filling them about halfway up the sides. Bake the cake layers for 35 to 40 minutes and the cake "dome" in the mixing bowl for 50 to 55 minutes, or until a cake tester or toothpick inserted in the center comes out clean. Remove and let cool completely.
Use a serrated knife to level the tops of the cake layers and the flat side of the cake dome (the dome will be inverted so that the rounded side forms the top of the cake). Measure an 8-inch cardboard cake round against the bottom of the dome and trim the round as needed so the cake sits just inside the cardboard.
Stack the cake layers, spreading about 1/2 cup of the milk chocolate buttercream between each layer. Set the stacked cake on an untrimmed 8-inch cardboard cake round. Spread a small amount of milk chocolate buttercream on the top cake layer and top with the trimmed cardboard cake round, pressing down gently to adhere. Add a dab of milk chocolate buttercream on top of the cake round and invert the domed cake, trimmed-side down, onto it. Using the serrated knife, trim the stacked cake layers below the dome at an angle to come to a slightly narrower bottom (this should look like the tapered sides of a cupcake). Spread milk chocolate buttercream evenly around the sides of the trimmed cake where the "cupcake wrapper" would be. Frost the top of the cake with Champagne buttercream, making a swirled and wavy pattern.
Cut the Pirouette cookies to the height of the "cupcake wrapper" and place next to each other around the outside of the cake. Sprinkle with multicolored sprinkles, top with a maraschino and serve!
Whisk the flour, baking powder and salt together in a medium bowl.
In another medium bowl, beat the eggs and sugar with an electric mixer until light and foamy, about 2 minutes. While beating, gradually pour in the melted butter and then the vanilla.
While mixing slowly, add half of the flour mixture. Then add all the milk, followed by the remaining flour mixture; take care not to overmix the batter.
Cream the butter in a stand mixer fitted with the paddle attachment (or in a large bowl if using a handheld electric mixer). On low speed, beat in the powdered sugar and then the salt. Add the vanilla, increase the speed to medium-high and beat until light and fluffy. Beat in 2 tablespoons milk; adjust the consistency with extra milk as desired.
Melt the chocolate, then fold it into the buttercream. Use immediately, or refrigerate in an airtight container up to 3 days. Allow to come to room temperature and beat on medium-high until smooth.
Cream the butter in a stand mixer fitted with the paddle attachment (or in a large bowl if using a handheld electric mixer). On low speed, beat in the powdered sugar and then the salt. Add the Champagne, increase the speed to medium-high and beat until light and fluffy. Use immediately, or refrigerate in an airtight container up to 3 days. Allow to come to room temperature and beat on medium-high until smooth.