Preheat oven to 350°F (175°C) and grease a 9×13-inch baking dish.

Cream butter and sugar: In a large bowl, beat the softened butter and granulated sugar together until light and fluffy.

Mix in eggs and sour cream: Beat in the eggs one at a time, followed by the sour cream and vanilla extract, mixing until well combined.

Combine dry ingredients: In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Incorporate dry into wet: Gradually add the dry ingredients to the wet mixture, stirring until just combined (avoid overmixing).

Fold in blueberries: Gently fold in the blueberries, being careful not to break them apart.

Layer the batter: Pour half of the batter into the prepared baking dish and spread evenly.

Add cinnamon sugar: In a small bowl, mix together the brown sugar and cinnamon. Sprinkle half over the batter.

Repeat layers: Pour the remaining batter on top, then sprinkle with the rest of the cinnamon sugar mixture.

Bake: Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Cool & serve: Allow the cake to cool slightly before slicing and serving. Enjoy!