Prepare the Streusel: In a medium bowl, whisk together the flour, brown sugar, cocoa powder, and cinnamon. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Set aside.

Preheat Oven and Pan: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch square baking pan or a round cake pan.

Mix the Dry Ingredients: In a large bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt.

Mix the Wet Ingredients: In a separate bowl, using an electric mixer, cream the softened butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, followed by the vanilla extract.

Combine and Bake: Alternately add the dry ingredients and buttermilk to the wet mixture, beginning and ending with the dry ingredients. Mix until just combined; do not overmix. Pour half of the cake batter into the prepared pan. Sprinkle half of the streusel topping over the batter. Pour the remaining batter over the top, then finish with the rest of the streusel.

Bake: Bake for 40-50 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 15 minutes before serving.