• Preheat oven to 350°F.
• Using a KitchenAid stand mixer or a handheld electric mixer, blend together butter, salt, sugars, baking soda, baking powder, and vanilla extract. 
  1 cup butter, softened, ¼ teaspoon salt, 1 cup granulated sugar, ½ cup light brown sugar, packed, 1 teaspoon baking soda, 1 teaspoon baking powder, 1 tablespoon vanilla extract
• Next, with mixer turning, add eggs one at a time, making sure that each is blended into the batter before adding the next one.
  3 large eggs
• In a separate bowl, using a spoon, mix together milk and sour cream. With mixer turning, alternate adding milk/sour cream mixture and flour to the batter.
  1 ¼ cups milk, ¾ cup sour cream, 3 ½ cups all purpose flour, sifted
• Pour half the cake batter into a greased 9 x 13 baking dish and spread it out evenly.

Filling Layer

• In a small bowl, using a spoon, mix together brown sugar, cinnamon, flour, and cocoa powder. Sprinkle generously over the first layer of cake batter. This will make the middle filling layer.
  ¾ cup light brown sugar, packed, 1 ½ teaspoons cinnamon, ¼ cup all purpose flour, 1 ½ teaspoons cocoa powder
• Spread the rest of the cake batter over the top. 

Crumb Topping

• Finally, in another bowl, mix together butter, sugars, cinnamon, and flour to make the crumb topping. Keep mixing it with a fork or spoon until crumbles form. Sprinkle over the top of the cake.
  6 tablespoons butter, melted, ¼ cup granulated sugar, ¾ cup light brown sugar, packed, 1 ½ teaspoons cinnamon, 1 cup all purpose flour
• Bake the coffee cake for 45 to 50 minutes, or until a toothpick comes out mostly clean.