Prepare Pan and Heat Oven. Preheat oven to 350 degrees Spray an 9x9-inch square baking pan with nonstick cooking spray. (For a thicker coffee cake, use an 8x8-inch pan.)

Make the Streusel. Stir together the brown sugar, baking flour, and cinnamon in a small bowl. (You’ll use the butter later.)

Prepare the Cake Batter. Whisk together the baking flour and baking powder in a small bowl.

Beat together the butter and cinnamon until light, about one minute on medium-high speed on a stand or handheld mixer.

Add the granulated sugar. Beat until a thick paste forms, about 45 seconds. Stop the mixer. Scrape down the bottom and sides of the bowl. Mix again for 15 seconds. (This mixes in the thick butter that was stuck to the bottom of the bowl.)

Stop the mixer. Add the eggs. Mix until combined. Stop the mixer. Scrap the bowl. Mix again for 15 seconds.

Add HALF the flour mixture. Mix until combined. Add HALF the sour cream. Mix until smooth. Repeat with the remaining flour and sour cream. Mix until batter is smooth.

Spread Batter into Pan and Add Filling. Spread half the batter into the prepared pan. Sprinkle half the streusel mixture over the batter.

Add Remaining Batter and Top with Streusel. Spread remaining batter over the filling. Stir the melted butter into the remaining streusel topping. Sprinkle the streusel topping evenly over the cake.

Bake and Cool. Bake until a cake tester inserted into the center of the cake comes out clean, about 45 minutes.

Cool for 20 minutes before serving. Store, wrapped, on the counter for up to three days.