• Preheat the oven to 350°F. Prepare two 8-inch round cake pans.
• Prepare the filling by mixing together the brown sugar, cinnamon, and cocoa powder. Set aside.

Cake

• In a large bowl, beat together the butter, salt, sugars, baking powder, and vanilla until well combined and smooth.
• Add the eggs one at a time, beating well after each addition.
• In a separate bowl, whisk together the sour cream or yogurt and milk till well combined.
• Add the flour to the butter mixture alternately with the milk/sour cream mixture, beating gently to combine.
• Pour/spread half the batter, roughly 1 1/3 cups, in each pan.
• Sprinkle the filling evenly atop the batter.
• Spread the remaining batter atop the filling. Use a table knife to gently swirl the filling into the batter, as though you were making a marble cake. Don't combine filling and batter thoroughly; just swirl the filling through the batter.
• Bake the cake until it's a dark golden brown around the edges; medium-golden with no light patches showing on top, 50 to 55 minutes for the 8" round pans. When pressed gently in the middle, the cake should spring back. If the cake starts to get too brown, cover with aluminum foil for remaining cooking time.
• Remove the cake from the oven and allow it to cool for 20 minutes before cutting and serving.

To assemble cake:

• Prepare glaze. You will want the glaze to be slightly firm, much like a thick pancake batter. The thicker the glaze is the most white it will appear, the runnier it is the more translucent it will become. Vanilla is options for purely vanity reasons. Omit if you prefer a bright white glaze.
• Place one layer of coffee cake on cake stand and cover in about 1/4 cup of glaze.
• Place other layer of coffee cake on top of that. Cover in about 1/4 cup of glaze, letting some spill over sides.
• Let glaze set for about 15 minutes or long enough that it is no longer wet to the tough.
• Starting at the center, place a donut ball on the cake. Nestle it into the glaze and make sure it is stuck.
• Start placing donut balls around center ball until you can covered the cake.
• Take remaining glaze and add up to 1 teaspoon more of milk. It will spill over the donut balls easier if it is runny. Pour about 1/4 cup of glaze over layer of donut balls letting it spill over and down sides of cake.
• Place another layer of donut balls on top of existing layer and add more glaze. Go as high as you are comfortable with.
• When all done pour remaining glaze over cake (or not, it just depends on how much glaze you want!) and then refrigerate until ready to serve. The fridge helps the glaze to set quickly so that the donut balls do not slide off.
• When read to serve, cut into cake with a sharp knife and then place piece on plate. Heat piece of cake for 15-30 seconds or until warm and gooey.