Preheat the oven to 350°F.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large bowl, cream together the butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

Stir in the vanilla extract.

Add the flour mixture to the butter mixture in three parts, alternating with the sour cream, and mixing until just combined.

Grease a 9-inch square baking dish with butter or cooking spray.

Pour half of the cake batter into the prepared baking dish.

Sprinkle the cinnamon sugar evenly over the batter.

Pour the remaining cake batter on top of the cinnamon sugar layer.

To make the streusel topping, combine the flour, brown sugar, granulated sugar, and cinnamon in a small bowl.

Pour the melted butter over the dry ingredients and stir until the mixture is crumbly.

Sprinkle the streusel topping over the cake batter.

Bake for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Let the cake cool for 10-15 minutes before slicing and serving.