Preheat the oven to 350℉. Line a 9X9-inch square baking pan with parchment paper or grease with non-stick spray or butter.

In a large bowl using an electric mixer, begin creaming together the softened butter, white sugar, and brown sugar. Add the eggs, vanilla, and sour cream and continue creaming for 1-2 minutes until incorporated.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt until combined.

Add the combined dry ingredients to the bowl with the butter mixture and mix at low speed just until incorporated and no streaks of flour remain.

Spread half of the batter into the bottom of the prepared pan. Stir 1 tablespoon of flour into the blueberries until coated. Add half of the flour-coated blueberries in an even layer on top of the batter followed by dropping the remaining batter by spoonful over and spreading to cover the blueberries. Top with the remaining blueberries in an even layer.

Stir together the melted butter, brown sugar, flour, and cinnamon to make the streusel mixture. Crumble the streusel mixture over the final layer of blueberries in an even layer.

Bake in the preheated oven for 45-60 minutes until a toothpick inserted into the center comes out clean. Let cool for 30 minutes before slicing to serve. Enjoy!