Preheat your oven to 350°F (175°C) and grease and flour a 9-inch square baking pan.

In a medium bowl, whisk together 2 cups of flour, 1 teaspoon of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of salt. Set aside.

In a large bowl, beat 1/2 cup softened butter and 1 cup granulated sugar together until light and fluffy, about 3-5 minutes.

Add 2 large eggs one at a time, beating well after each addition, then mix in 1 teaspoon of vanilla extract.

Gradually alternate adding the dry ingredients and 1 cup of sour cream to the butter mixture, starting and ending with the dry ingredients. Mix until just combined.

In a small bowl, combine 1/2 cup brown sugar and 2 teaspoons ground cinnamon for the swirl layer.

Pour half of the batter into the prepared pan and smooth out the top.

Drizzle half of the melted butter over this layer and sprinkle the cinnamon sugar mixture evenly on top.

Pour the remaining batter over the cinnamon layer, smoothing it out and drizzle the remaining melted butter over the top.

In a small bowl, mix together 1/2 cup of flour, 1/4 cup brown sugar, and 1/4 cup granulated sugar for the crumb topping.

Cut in 1/4 cup cold, cubed butter until the mixture resembles coarse crumbs.

Sprinkle the crumb topping evenly over the cake batter.

Bake for 40-45 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan on a wire rack before slicing.