Preheat oven to 350°F (177°C). Grease three 8-inch round cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)

Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed until combined and creamy, about 3 minutes. Scrape down the sides of the bowl with a silicone spatula as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg whites. Beat on high speed until combined, about 2 minutes. Then beat in the sour cream and vanilla extract, again on high speed, until the mixture comes together. Scrape down the sides as needed. Add the dry ingredients and turn the mixer on low speed. With the mixer running on low, slowly pour in the milk until combined. Do not overmix. You may need to whisk it all by hand to make sure there are no lumps or dry pockets at the bottom of the bowl. The batter will be slightly thick. Fold in the mini chocolate chips.

Pour batter evenly into cake pans. Weigh them to ensure accuracy, if desired. Bake for 24–26 minutes or until the cakes are baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done. Or you can lightly poke the top of the cake with your finger. If it bounces back, it’s done.

Allow cakes to cool in the pans set on a cooling rack for 1 hour, then remove them from the pans and let the cakes finish cooling directly on the rack. The cakes must be completely cool before frosting and assembling.

(Note that if you want to pipe plain buttercream on top of the cake like in the photos and video, set around 3/4 cup (about 180g) aside before adding the chocolate chips.) In a large bowl using a handheld mixer or stand mixer fitted with a paddle attachment, beat the butter on medium to medium-high speed until smooth and creamy, about 2 minutes. Add confectioners’ sugar, heavy cream, vanilla extract, and salt. Start the mixer on low speed, then gradually increase to high speed and beat until whipped and fluffy, about 4 minutes. Fold in the mini chocolate chips, either by hand with a spatula, or you can beat them in on low speed until they’re evenly dispersed throughout the frosting.

Using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard the scraps, or you can save them to crumble on top of the finished cake. Place 1 cake layer on your cake stand, cake turntable, or serving plate. Evenly cover the top with about 1 cup of frosting. Top with 2nd cake layer, upside down, and evenly cover the top with another 1 cup of frosting. Top with the third cake layer, either right side up or upside down. Spread the remaining frosting all over the top and sides. I use and recommend a large, straight icing spatula to apply the frosting.

Refrigerate the cake for at least 20 minutes, or up to 8 hours. The frosting must be cold when you add the chocolate drip on top.

Place chopped chocolate in a medium heat-proof bowl with a pour spout (I use a 2-cup glass liquid measuring cup). Heat the cream in a small saucepan over medium heat until it begins to gently simmer. (Do not let it come to a rapid boil—that’s too hot!) Pour over chocolate, then let it sit for 2–3 minutes to gently soften the chocolate. With a metal spoon or small rubber spatula, very slowly stir until chocolate has melted and mixture is smooth. The finer you chopped the chocolate, the quicker it will melt with the cream. If it’s not melting, do not microwave it. If needed, see Troubleshooting Chocolate Ganache. Once ganache mixture is smooth, transfer it into a squeeze bottle.

Remove the cake from the refrigerator. Working in a zigzag motion, gently squeeze the warm ganache around the edge of the cake, turning the cake as you go. See video tutorial below as a visual. The ganache should drip down the sides and set as it does. When you’ve gone around the whole cake, squeeze remaining ganache on top of the cake, and smooth it into an even layer with an offset spatula. Feel free to garnish with fresh berries, chocolate chips, or leftover cake crumbles from leveling the tops of the cakes. Chill, uncovered, for at least 20 minutes, or up to 6 hours before slicing and serving. Once the ganache has chilled, if you set aside buttercream in step 6, you can pipe it on. I used Wilton 1M piping tip.

Cake can be served at room temperature or chilled. I prefer it at room temperature. Cover leftover cake tightly and store in the refrigerator for up to 5 days. I like using a cake carrier for storing and transporting.