In a medium bowl, mix the dry streusel ingredients and then cut in the cold butter (this can also be done in a food processor) and set aside.
Preheat the oven to 375 degrees.
In a large mixer bowl, cream the butter and sugar.
Add the egg and mix.
In another bowl, mix the dry ingredients.
Add the dry ingredients to the butter mixture alternating with the milk with the mixer on low speed.
Add the vanilla and mix together.
Fold the blueberries in by hand with a spoon, the batter will be very thick.
Spread the batter evenly in a 8 inch square pan sprayed with cooking spray.
Spread the streusel over the cake batter.
Bake for 40 mins or until a toothpick comes out clean from the center of the cake.
Let cake cool for 15 minutes, then cut into squares and serve warm.