Preheat oven to 350°
Line a 8x8 pan with parchment paper, leaving a 1” edge higher than the pan; spray with cooking spray.
Mix flour, baking powder and salt together and set aside. Beat the butter and sugar in a medium bowl until pale and fluffy, about 2 minutes. Add in the egg and beat to mix. Alternating with flour and milk, starting with the flour, add flour mixture and milk to butter-sugar mixture. When well mixed, fold in blueberries. If using frozen berries, do not thaw. Spread batter into pan.
For topping mix flour, butter, sugar and cinnamon, crumbling with your fingers. Scatter the crumbs over the batter. Bake for about 1 hour and 10 minutes or until a toothpick inserted in the center of the cake comes out clean. Cool for about 20 minutes in the pan and then lift cake out used parchment paper edges. Cool slightly before cutting.