Black & White Pound Cake

Directions

1. Set a rack in the lower-middle position and heat the oven to 350°F. Grease a 9x5-inch metal loaf pan with butter.
2. For the streusel: In a medium bowl, whisk together the flour, brown sugar, cocoa, and salt. Add the cubed butter and rub the mixture together with your fingers until it comes together into clumps; set aside.
3. For the pound cake: In the bowl of a stand mixer fitted with a paddle attachment, beat the butter, sugar, baking powder, and kosher salt on medium-high speed until pale and very fluffy, stopping once during mixing to scrape down the paddle and bowl, 6 to 8 minutes total. (You might think it’s done before that time, but keep going all the way.)
4. Scrape down the paddle and sides of the bowl. On medium-high speed, beat in the eggs and yolk one at a time, scraping down the paddle and the bowl after each addition. The batter should look very fluffy, creamy, and emulsified (if not, your eggs or butter may have been not at room temperature—let the mixture come to room temperature then try mixing it again).
5. Scrape down the paddle and sides of the bowl. In a small bowl, stir together the sour cream and vanilla until lump-free and totally smooth.
6. Add half of the sour cream and mix on low until just incorporated, about 15 seconds. Add half of the flour and mix until just incorporated, about 15 seconds. Repeat with the remaining sour cream and flour. Using a flexible rubber spatula, scrape down the paddle and the bowl and mix the batter a few times to make sure everything is evenly combined.
7. Transfer about half the batter to a medium bowl. Sift over the cocoa powder and stir into the batter along with the milk or water.
8. Add half the cocoa batter and half the vanilla batter to the pan in alternating dollops (watch the video for a visual!). Top with half the streusel and repeat dolloping the remaining batter.
9. With a butter knife or offset spatula, swirl the knife through the batter to make sure it is evenly distributed into the pan with no big air pockets and give the pan a few swift taps against the counter. Wet a butter knife and use it to slice down the center of the loaf cake (this ensures an even crack right down the middle of the cake). Top with remaining streusel, lightly pressing it into the batter.
10. Bake until the crust is deeply browned, the loaf rises and splits, and the cake feels firm and set when you gently press the top, 65 to 75 minutes. (This is a very moist cake and it is better to overbake rather than underbake. If the crust is looking very dark partway through, set a wire rack just above the loaf pan and place a rimmed baking sheet on it to provide a shield.)
11. Cool the cake in the pan for 10 minutes, then run an offset spatula or butter knife around the sides to loosen. Tip the cake into your hand, then place on a wire rack to fully cool before slicing.