Prepare Cinnamon Filling: In a small bowl, whisk together brown sugar, flour, and ground cinnamon. Set aside.

Make Streusel Topping: In a medium bowl, whisk together flour, brown sugar, cinnamon, and salt. Add diced cold butter and use a pastry blender to incorporate until crumbly. Set aside.

Prepare Cake Batter: Preheat oven to 350°F. Grease and line an 8x8-inch square baking pan with parchment paper. In a medium bowl, whisk together flour, baking powder, and salt. In a large mixing bowl, beat softened butter, granulated sugar, and brown sugar until light and fluffy. Add eggs one at a time, then mix in sour cream and vanilla extract. Gradually add dry ingredients to wet ingredients, mixing until just combined.

Assemble Cake: Spread half of the batter into prepared pan. Sprinkle Cinnamon Filling evenly over batter. Dollop remaining batter over filling, then spread it out evenly. Sprinkle Streusel Topping over batter.

Bake: Place pan in preheated oven and bake for 55 minutes or until a toothpick inserted in the center comes out clean. Allow cake to cool in pan for 30 minutes before serving.

Optional Glaze: Whisk together powdered sugar, milk, and vanilla extract until smooth. Drizzle over cooled coffee cake before or after slicing.