Gather the ingredients.
Grease and flour a 9-inch square baking pan. Heat oven to 375 F (190 C/Gas 5).
In a mixing bowl with an electric mixer beat the 4 tablespoons of butter with 3/4 cup granulated sugar until creamy; add the egg and vanilla and continue to beat until smooth and well blended.
Combine the 2 cups of flour, baking powder, and salt in a small bowl.
Add the flour mixture to the butter mixture, alternating with 1/2 cup of milk and beating until smooth after each addition. Gently fold the blueberries into the batter.
Spread the batter in the prepared baking pan.
In a medium bowl, combine the 4 tablespoons of room temperature butter with the brown sugar, 1/3 cup of flour, and 1/2 teaspoon of cinnamon; blend well with a pastry blender, fork, or fingers to form crumbs.
Sprinkle topping crumbs over the batter. Bake the cake in the preheated oven for 35 minutes.
Remove the cake from the oven and allow it to cool. Slice it into squares and serve.