Preheat the oven to 350°F. Grease and line 3 6-inch cake pans with parchment paper. Set them aside.
Place the chocolate chips, milk, and sour cream in a heat-proof bowl. Sift the cocoa powder into the bowl and set it aside.
56g (2oz) semi-sweet chocolate chips, ½ cup milk, ¼ cup sour cream, ½ cup black cocoa powder **
Place a small saucepan fill with 3-inches of water over high heat. Bring the water to a boil and reduce it to a simmer. Place the heat-proof bowl over the mouth of the pot and stir until the chocolate melts and all the ingredients are thoroughly integrated. Take the mixture off of the heat and set it aside to cool.
While the chocolate is cooling, sift the flour, baking powder, and soda into a medium-sized bowl. Add the salt and whisk to combine. Set it aside.
2 cups cake flour, 1 tsp baking powder, 1 tsp baking soda, 1 tsp kosher salt
In the bowl of a stand mixer fitted with a paddle attachment place the butter and sugar. **** Cream the sugar into the butter until light and fluffy. Add the eggs to the mixture one at a time, scraping down the sides of the bowl as needed. Add the vanilla and mix until fully integrated.
1 cup unsalted butter, 1⅓ cup granulated sugar, 4 large eggs, 2 tsp vanilla extract
Pour the chocolate mixture into the egg mixture and mix until uniform. Add half of the flour mixture and mix until no streaks remain. Repeat with the remaining flour mixture.
Fill the prepared pans with the cake batter. Add roughly 400g of batter to each pan. Smooth tops with an offset spatula and transfer the cake to the oven. Bake for 50 minutes, rotating the pans once halfway through.
Take the cakes out of the oven and remove them from their pans. Place the cakes on a cooling rack and set them aside to cool for two hours.