Preheat the oven to 350 degrees F and grease a 9x9 square baking pan. Set the pan aside.
Make the cinnamon filling first. Whisk all of the ingredients for the cinnamon filling together in a small bowl then set aside.
¾ cup brown sugar, ¾ cup all purpose flour, 1 Tablespoon ground cinnamon
Next, make the topping. Add all of the ingredients for the topping to a food processor and pulse until the mix looks coarse and crumbly. Set the topping aside as well.
5 Tablespoons butter, ¾ cup all purpose flour, ½ cup brown sugar, 1 Tablespoon ground cinnamon, ¼ teaspoon salt
Make the cake batter by beating the butter, brown sugar and white sugar together until light and fluffy. Scrape down the sides of the mixing bowl well to ensure everything is evenly blended.
½ cup butter, ¼ cup brown sugar, ½ cup granulated sugar
Add the vanilla and egg to the mixing bowl and beat again to make a smooth batter.
1 teaspoon vanilla extract, 1 egg
In a separate bowl, whisk together the flour, baking powder and salt.
2 cups all purpose flour, 2 teaspoons baking powder, ¼ teaspoon salt
In another bowl, whisk together the milk and sour cream.
½ cup whole milk, ½ cup sour cream
Add about ⅓ of the flour mix into the mixing bowl with the butter and egg mix. Let the batter come together and then add half of the milk mix. Add another ⅓ of the flour, mix well, then add the rest of the milk. Add the remaining flour to the mixer and stir until smooth and thick.
Pour half of the cake batter into the prepared pan. Spread the batter evenly in the pan, making sure it gets into the corners.
Sprinkle the cinnamon filling over the cake batter in the pan, making it as even as possible. Try to coat the entire pan with a nice layer of cinnamon filling.
Pour the remaining cake batter into the pan carefully, trying not to mix in the cinnamon filling layer too much- you really want that cinnamon layer to stay in the middle of the cake batter!
Gently spread the cake batter in the pan so it is even.
Sprinkle the crumb topping over the batter evenly.
Bake the coffee cake for 40 to 50 minutes. The center of the cake should be set and a toothpick should come out cleanly when stuck in the center of the cake.
Let the coffee cake cool in the pan for about 15 minutes before slicing and serving.