Preheat oven to 375 degrees (350 degrees if using dark metal pan)
Make the crumb topping by cutting the butter into the dry ingredients with a pastry blender or fork. The mixture will be coarse. Set aside.
Begin making the cake batter by creaming the butter and sugar with a hand or stand mixer until light and fluffy.
Add egg and blend until incorporated.
Add the vanilla and milk and blend until mixed.
In a separate bowl, sift the dry ingredients (flour, baking powder, and salt).
Add dry ingredients to wet and mix until just combined.
Fold in blueberries.
Pour the batter into a well-greased 9-inch square cake pan. Sprinkle the crumb topping evenly over the batter.
Bake for 40 to 45 minutes or until a cake tester comes out clean. (If using a dark metal pan, start checking at 30 to 35 minutes)