Yield: 6-inch cake
Preheat oven to 350 degrees F. Line three 6-inch round cake pans with parchment paper. Grease lightly and set aside.
Whisk together flour, baking powder, and salt. Set aside. In the bowl of a stand mixer fitted with a paddle attachment, combined butter, coconut oil, and sugar. Add eggs and beat until mixture is pale in color. Scrape down bowl as needed to ensure thorough mixing.
Alternate adding flour mixture and milk, beginning and ending with the flour. Mix on low speed until combined. Fold in chopped cookie sandwiches.
Divide batter among the three prepared cake pans. Level batter and bake for 35-40 minutes until the cakes are golden brown and a toothpick inserted in center of cake comes out clean. Let cool in pan for 5 minutes before unmolding. Allow to cool to room temperature on wire rack.
1. In the bowl of a stand mixer, whisk together egg whites and sugar. Place bowl over a pot of simmering water to create a double boiler. Make sure simmering water does not touch the bottom of the bowl. Whisk sugar egg mixture until it reaches a temperature of 160 degrees F.
Alternatively, if you don't have a thermometer, heat until all the sugar has melted. You can test this by dipping your pointer finger into the warm mixture and rubbing it against your thumb. The mixture should be smooth. You should not be able to feel the granules of sugar.
Return bowl to the stand mixer. With the whisk attachment, beat mixture at medium speed until it has cooled to room temperature and mixture is at medium stiff peaks. Reduce speed to low and add butter one tablespoon at a time. Once all the butter has been incorporated, add a pinch of salt. Increase speed to medium-high and continue to beat until buttercream is light and fluffy.
If desired, add a few drops of red gel food coloring to achieve pink color. Fold in chopped sandwich cookies. Set aside until ready to assemble cake.
In the bowl of a stand mixer fitted with a paddle attachment (or using a hand mixer), cream butter until smooth. Sift together powdered sugar and cocoa powder. Add to bowl in three additions. Mix on low speed after each addition. Add salt and milk. Mix until combined. Adjust consistency according to preference. If buttercream is too thick, add milk 1 Tbsp at a time until desired consistency is achieved. If buttercream is too thin, add 1/4 cup sifted powder sugar (up to 3/4 cup). Set aside until ready to use.
Place one cake round on cake stand, or serving dish of choice. Spread a thin layer of Swiss buttercream. Top with second cake round followed with thin layer of Swiss buttercream. Finish with remaining cake round.
Spread Swiss buttercream all over cake to seal in the crumbs (crumb coat). Let chill in the refrigerator for at least 15 minutes. Once cake is chilled, spread chocolate buttercream over cake.
Pour melted chocolate over finished cake. Decorate cake with additional piped buttercream and chopped cookie sandwiches.