Directions:

Prep your oven: Preheat the oven to 350°F (175°C). Grease an 8×8-inch baking dish.
Mix the dry ingredients: In a bowl, whisk together flour, baking powder, and salt.
Cream the butter & sugar: In another bowl, beat the softened butter and granulated sugar until light and fluffy. Add the egg and vanilla extract, mixing well.
Alternate wet & dry ingredients: Gradually add the flour mixture, alternating with milk, mixing until smooth. Fold in the blueberries.
Pour into dish: Pour the batter into the prepared baking dish.
Prepare the topping: Combine brown sugar, flour, cinnamon, and chilled butter pieces. Sprinkle this mixture evenly over the batter.
Bake: Bake for 45-50 minutes, or until a toothpick inserted in the center comes out clean and the top is golden.