Preheat the oven to 375° F. Line a 9x9 baking dish with parchment paper and set aside.
In a medium bowl whisk together the flour, baking powder, and salt. Set aside.
In a separate large bowl, use an electric mixer to cream sugar and butter together until smooth, about 1-2 minutes. Add the egg and mix until combined.
Add the flour mixture and milk in alternating additions, starting and ending with the flour mixture. Mix until just combined, be careful not to overmix here.
Coat the blueberries with 1 tablespoon flour and gently fold them into the batter. Spread the batter into the prepared baking pan (it will be thick).
Streusel Topping:
In a bowl, cut the butter and other ingredients together with a fork until it becomes crumbly. Sprinkle evenly on top of the cake.
Bake the cake at 375° for 30-35 minutes or until a toothpick comes out clean. Allow the cake to cool in the pan for about 10 minutes, and then remove it to a wire cooling rack.
Video
Notes
Fresh blueberries are best. Tossing them with a spoonful of flour keeps them evenly distributed throughout the cake.
If using frozen berries, there is no need to defrost first but your cake may need a few extra minutes to finish baking.