Make the Cake Layers:
Preheat oven to 350°F (175°C). Grease and line three 8-inch cake pans with parchment paper.
In a bowl, whisk together cake flour, baking powder, and salt. Set aside.
In a separate bowl, beat butter and sugar on high speed until light and fluffy (about 3 minutes).
Add egg whites one at a time, mixing well after each addition.
Mix in sour cream and vanilla extract until smooth.
With the mixer on low, alternate adding the dry ingredients and milk, mixing until just combined.
Gently fold in the mini chocolate chips.
Divide batter evenly among the cake pans and bake for 22-25 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
Make the Buttercream Frosting:
Beat butter until creamy and smooth.
Gradually add powdered sugar, mixing well after each addition.
Stir in vanilla extract.
Add heavy cream, 1 tablespoon at a time, and beat until fluffy.
Assemble the Cake:
Place the first cake layer on a cake stand and spread buttercream on top.
Repeat with the second and third layers, then frost the entire cake.
Smooth out the buttercream with an offset spatula.
Make the Chocolate Ganache Drip:
Heat heavy cream until steaming (but not boiling).
Pour over chopped chocolate and let sit for 1 minute.
Stir until smooth and let cool slightly before using.
Apply the Ganache Drip:
Chill the frosted cake for 20 minutes before adding ganache.
Using a spoon or squeeze bottle, drizzle ganache over the edges of the cake.
Pour the remaining ganache on top and smooth it out.