From Cook's Illustrated

• 4 oz chopped unsweetened chocolate
• ¼ cup Dutch-processed cocoa powder
• 1 ¼ cups boiling water
• ¾ cup all-purpose flour
• ¾ cup cake flour
• 1 tsp baking soda
• ¼ tsp salt
• 2 sticks softened unsaltened butter
• 1 ½ cups packed dark brown sugar
• 3 room temperature eggs
• ½ cup sour cream
• 1 tsp vanilla extract

Preheat the oven to 350 with the racks at upper and lower middle. Grease three 8-inch round cake pans with butter and line the bottoms with parchment paper.

Whisk the boiling water and chocolates together. Sift the flours, baking soda, and salt together.

Cream the butter at medium-high speed in a stand mixer (about 1 minute). Add the brown sugar and beat on high for three minutes, then scrape down the bowl. Beat the eggs in at medium high one at a time, waiting 30 seconds between eggs. Beat in the sour cream and vanilla at medium for 10 seconds, then scrape down the bowl again. Add in a third of the flour, half the chocolate, flour, chocolate, and flour on low. Scrape the bowl and mix gently to combine.

Put a third of the batter in each cake pan and smooth to the edges of the pans. Bake until a toothpick comes out clean, 20 to 23 minutes (make sure to check the racks separately; in our oven, the pan on the lower rack took noticeably longer). Cool for 15-20 minutes in the pans, then run a knife around the edges and remove the cakes onto a plate. Peel off the parchment paper and flip the cakes back over onto a lightly greased cooling rack. Cool completely.