Preheat the oven: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round or square baking pan.
Prepare the cinnamon swirl: In a small bowl, mix the brown sugar and cinnamon together for the swirl. Set aside.
Make the streusel topping: In another small bowl, combine the flour, sugar, melted butter, and cinnamon for the topping. Stir until crumbly and set aside.
Prepare the cake batter: In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a separate large mixing bowl, cream together the butter and sugar until light and fluffy (about 2-3 minutes).
Beat in the eggs one at a time, then add the vanilla extract.
Gradually mix in the dry ingredients, alternating with the sour cream, starting and ending with the dry ingredients. Mix until just combined.
Assemble the cake:
Pour half of the cake batter into the prepared pan and spread it evenly.
Sprinkle the cinnamon swirl mixture over the batter.
Add the remaining batter on top and spread evenly.
Sprinkle the streusel topping over the top of the cake.
Bake the cake: Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Cool and serve: Let the coffee cake cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely. Slice and serve.