Preheat the oven to 350°F (175°C) and grease a 9×13-inch baking pan.

Make the streusel topping: In a small bowl, combine the flour, brown sugar, cinnamon, and salt. Mash in the softened butter until the mixture is crumbly. Set aside.

Prepare the cinnamon layer: In another small bowl, mix together the sugar, flour, and cinnamon. Set aside.

Make the cake batter: In the bowl of a stand mixer, combine the flour, sugar, baking powder, baking soda, and salt. Mix on low speed. Add the softened butter and beat on medium-low until the mixture resembles moist sand.

Combine the wet ingredients: In a medium bowl, whisk together the sour cream, milk, eggs, and vanilla. Add this mixture to the dry ingredients and beat until smooth. Don’t worry if a few lumps remain.

Assemble the cake: Spread half of the batter into the prepared pan. Sprinkle half of the cinnamon-sugar layer evenly over the batter. Add the remaining batter on top, spreading it gently. Finish by sprinkling the streusel topping evenly over the entire cake.

Bake: Place the pan in the oven and bake for 55 minutes, or until a toothpick inserted into the center comes out clean.

Cool and serve: Let the coffee cake cool in the pan for 10 minutes before transferring it to a wire rack. Allow it to cool completely, then slice and enjoy!