• Preheat the oven to 350 degrees. Spray a 9 by 13 inch baking dish generously with cooking spray. Set aside.

To Make Streusel Topping

• In a small bowl, whisk together the dry ingredients for the streusel.
  1 cup brown sugar, 1 cup all purpose flour, 1 teaspoon cinnamon
• Using a pastry cutter or a fork, cut the butter into the dry ingredients until the mixture is crumbly. Set aside.
  8 tablespoons unsalted butter

To Make the Filling

• Whisk together the ingredients for the filling and set aside.
  1/4 cup brown sugar, 1/4 cup flour, 1/2 teaspoon cinnamon

For the Cake Batter

• In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Set aside.
  2 cups all purpose flour, 2 teaspoons baking powder, 1 teaspoon baking soda, 1 teaspoon cinnamon, 1/2 teaspoon salt
• In a stand mixer fitted with the paddle attachment or in a large bowl using a hand mixer, beat the butter and granulated sugar together until light and fluffy, about 3 to 6 minutes.
  1/2 cup unsalted butter, 1 cup granulated sugar
• Add in the eggs, one at a time. Now beat in the vanilla extract. Scrape down the sides, and beat in the sour cream until the mixture is smooth and combined. Scrape down the sides again. Finally, beat in the milk.
  3 eggs, 2 teaspoons vanilla extract, 3/4 cup sour cream, 1/4 cup milk
• Add in the flour mixture, and beat on low for a few moments until you can beat on high until the mixture is just combined.
• Pour half of the batter into the prepared pan. Top with the filling. Carefully add the rest of the batter. Top with the streusel, and bake for 40 to 45 minutes or until a toothpick inserted in the middle comes out with just a few crumbs.

For Icing

• Before serving, whisk together the powdered sugar and milk. If necessary, add a little more milk until the mixture is runny. Drizzle over the coffee cake.
  1/2 cup powdered sugar, 1 tablespoon milk