In the Oven:

Streusel:
1/3 c. all-purpose flour
1/3 c. granulated white sugar
1/2 tsp ground cinnamon
1/4 c. cold unsalted butter, cut into pieces

Cake Batter:
1 c. all-purpose flour
1 tsp baking powder
1/8 tsp. salt
1/4 c. unsalted butter, room temperature
1/2 c. granulated white sugar
1 large egg
1/2 tsp. pure vanilla extract
1/3 c. milk
2 c. fresh or frozen blueberries

Preheat oven to 350 degrees F. Grease an 8 x 8 inch square pan or an 8 inch round cake pan.

For streusel topping: In a large bowl, mix together the flour, sugar, and ground cinnamon. Cut in the butter with a pastry blender or fork until it resembles coarse crumbs. Set aside while you make the cake batter.

In a separate bowl whisk together the flour, baking powder, and salt. Set aside.

With an electric beater, beat the butter until smooth. Add the sugar and beat until light and fluffy. Add the egg and vanilla and beat until incorporated. Add the flour mixture, alternately with the milk, and beat only until combined. Spread the batter onto the bottom of the prepared pan, smoothing the top.
Evenly arrange the blueberries on top of the cake batter and then sprinkle with the streusel topping.

Bake for about 40 - 50 minutes or until a toothpick inserted in the center of the cake comes out clean. Remove from oven and place on a wire rack to cool slightly.

Serve warm or at room temperature.

Makes 9 servings.

Note: I doubled and put in a 9x13...it's in the oven now, so we'll see how that goes! But it looked like it would fit and be fine. :)