1. For the streusel add the flour, brown sugar, and black cocoa to a bowl and mix to combine. Add the cold butter in small pieces and work with your fingers until streusel form. Place in the fridge until needed.

2. Preheat the oven to 350°F (180°C). Line a 9×5 inches (23x13cm) loaf pan with a piece of baking parchment and set aside.

3. Add the butter and sugar to a large bowl and mix on high speed for about 4-5 minutes until light and fluffy. Add the eggs one after another and mix well after each addition. Mix the flour, baking powder, and salt. Add together with the sour cream and vanilla extract to the bowl and mix on low until just combined. Transfer about half of the batter to a bowl. Add the cocoa powder and milk to the batter in the large bowl and mix on low until well combined. Add dollops of both batters alternating into the prepared loaf pan – it should look like a checkerboard. Sprinkle about half of the streusel on top and repeat the process with the dollops of batter alternating to get a second layer. It will look best if you alternate the colors now so there are different colors on top of each other. Smooth out the top and sprinkle with the remaining streusel. Bake for 65-70 minutes or until a wooden skewer inserted into the center of the cake comes out clean. Remove from the oven and let cool down in the loaf pan for several minutes, then lift out of the pan with the parchment paper and let cool down completely on a wire rack.