Chocolate filling: 3 tbsp (33g) ground dark chocolate
2 tbsp (20g) brown sugar
1 tbsp (12g) sugar
1 tsp cinnamon
Cake batter: 2 cups (260g) cake flour, sifted
¾ cup (170g) sugar
1 tsp baking powder
½ tsp salt
¼ tsp baking soda
8 tbsp (112g) butter
1 egg + 2 egg yolks
¾ cup (200g) buttermilk
2 tbsp (22g) canola oil
2 tsp (10g) vanilla extract
Recipe
Make the streusel: grind cocoa nibs, flour, sugar, cinnamon, and salt in a food processor to a coarse powder. Blend in cold butter until crumbly. Refrigerate.
Mix chocolate filling ingredients together and set aside.
Preheat oven to 350°F. Prepare an 8-inch square pan with non-stick spray and parchment.
Combine cake flour, sugar, baking powder, salt, and baking soda in a mixer. Blend in butter until coarse and crumbly. Combine eggs, buttermilk, oil, and vanilla; add to dry mixture. Mix 15 seconds to incorporate, then 30 more seconds to develop structure.
Spread half the batter into prepared pan. Sprinkle chocolate filling over. Cover with remaining batter.
Crumble the chilled streusel over the top. Bake about 45 minutes. Cool completely before serving.