Preheat oven to 350°F.
Grease and line an 8x8 square cake pan.
To make streusel, combine 6 tbsp cold butter, 1/3 cup white sugar, 1/3 cup Demerara sugar, 1/3 cup flour, 1/2 tsp cinnamon, 1/2 tsp salt, and 2 tbsp cocoa powder. Mix with a fork until sandy and some medium clumps remain.
In a stand mixer with a paddle attachment, mix 6 tbsp soft butter and 1 cup sugar on medium for 4-5 minutes until fluffy.
In a pyrex measuring cup, whisk together egg, egg yolk, vanilla extract, coffee extract, sour cream, and buttermilk.
In a large bowl, sift 1 1/3 cups flour, 1/3 cup cocoa powder, baking powder, and salt.
With the stand mixer on low, alternate adding 1/3 of the flour mixture at a time with 1/3 of the egg and milk mixture. You will end with a thick chocolate cake batter.
Transfer half the cake batter to prepared pan. Then, sprinkle over half of your streusel mixture. Place the rest of your cake batter on top and smooth. Then, top with the rest of the streusel mixture.
Bake for 75 - 90 minutes until a cake tester or toothpick comes out clean. Slice and serve.