Preheat oven: Move your oven rack to the middle position. Preheat the oven to 350°F (180°C).

Prepare pans: Butter or oil the insides of three 6" cake pans and line the bottoms with a circle of parchment paper.

Bloom cocoa powder: In a large mixing bowl, add the black cocoa, chopped dark chocolate, brown sugar, salt, and boiling hot water. Whisk until thoroughly combined. Set aside your bowl for at least 5 minutes for the chocolate flavor to deepen and bloom.

Whisk in rest of ingredients: Add oil and sour cream to the bowl of chocolate and whisk. Keep whisking until it's slightly thicker, glossy, and thoroughly combined. Whisk in the all-purpose flour and baking powder until no traces of flour remain. Whisk in the yolks and eggs for about a minute until the batter is thoroughly mixed and velvety black.

Pour batter into pans: Evenly distribute the batter into the prepared pans. You can wait about 5-10 minutes after pouring the batter in your pans and then tap the pans on the counter a few times to get super smooth cake tops.

Bake and cool: Bake for about 23-28 mins. Start checking for doneness at 20 minutes. Move the pans to a cooling rack and let the cake cool slightly - about 5-10 minutes.

Remove and wrap cakes: Loosen the edges of the cakes using an offset spatula or butter knife, and flip the cakes out onto a cooling rack so you can peel off the parchment bottom. Wrap cakes in plastic wrap until ready to frost.