To prepare cake, preheat oven to 350F. Grease three 5-inch cake pans and line bottoms with parchment paper. Set aside.
In a large bowl, sift together flour, baking powder and salt.
In a standing mixer with a paddle attachment, beat the butter on medium speed until light and fluffy. Add sugar, egg whites and vanilla, continue beating until well combined.
With the mixer on low, add the flour mixture, in 3 additions, and buttermilk, in two additions, alternately, starting and ending with the flour. Mix until combined.
Divide batter evenly among the prepared cake pans. Bake for 18 to 20 minutes. Remove from oven and let cakes cool completely.
Run a pairing knife around the edge of the cake to loosen it from the pan. Peel off the parchment paper circles.
To prepare raspberry compote, cook raspberries, cornstarch, sugar and water in a small saucepan until the berries start to break down, about 10 minutes.
Lower the heat and simmer until the compote is thick, about 5 minutes. Let cool slightly before pushing the compote through a fine sieve to remove the seeded pulp. Let compote cool completely.
To prepare buttercream, beat butter for a few minutes in a standing mixer with the paddle attachment on medium speed. Add powdered sugar and turn your mixer on low, beat until the sugar incorporates with the butter. Add vanilla and heavy cream, beat on medium speed for another 3 minutes, or until buttercream becomes light and fluffy.
To assemble the cake, place one cake layer on your serving plate and spread a layer of buttercream, followed by a layer of compote. Place another cake layer on top and repeat the same process. Place the last layer of cake on top. Frost the top and sides of the cake with buttercream. Cover and chill for 30 minutes.
To prepare ganache, place the chocolate and heavy cream in a medium heatproof bowl over a pot of simmering water. Stir the mixture using a wooden spoon until melted and smooth. Let cool slightly. Tint the ganache with a drop of colouring paste.
Remove cakes from the refrigerator. Pour ganache over the top of the cake. Use an offset spatula to gently push the ganache over the sides of the cake to create the dripping effect.
Let ganache set for 5 to 10 minutes. Garnish as desired.