Preheat oven to 375°F and grease a 9x9 baking dish.

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

In another bowl, beat the butter and sugar until fluffy. Beat in the egg and vanilla.

Alternately mix in the flour mixture and milk, blending just until combined.

Fold in the blueberries gently.

Spread the batter evenly into the prepared pan.

Make the streusel by mixing the sugar, flour, cinnamon, and cubed butter until crumbly. Sprinkle over the batter.

Bake for 35-40 minutes until a toothpick inserted in the center comes out clean.

Allow to cool at least 10 minutes before serving warm or at room temperature.