Preheat the oven to 350°F and grease an 8x8-inch baking dish with nonstick cooking spray.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, beat together the melted butter and sugar until smooth and creamy. Mix in the cinnamon.
Add the eggs one at a time, mixing well after each addition.
Stir in the sour cream and vanilla extract until fully combined.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, and mix until just combined. Be careful not to over mix the batter.
In a small bowl, stir together the sugar and cinnamon for the topping.
Pour half of the batter into the prepared baking dish and spread it evenly. Sprinkle half of the cinnamon-sugar mixture over the batter.
Spread the remaining batter on top and finish with the remaining cinnamon-sugar mixture.
Bake for 25 to 30 minutes, or until the center is set and a toothpick inserted into the middle comes out clean.
Allow the cake to cool for 10 to 15 minutes before slicing and serving.