01 - Preheat oven to 350°F. Grease a 9-inch square baking pan or line with parchment paper.
02 - In a medium bowl, whisk together all-purpose flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat softened butter and granulated sugar with an electric mixer until light and fluffy. Incorporate the egg.
04 - Add flour mixture to creamed butter in three portions, alternating with milk, beginning and ending with flour. Blend until just combined.
05 - Gently fold in blueberries. Evenly spread the batter into the prepared pan using a rubber spatula.
06 - In a small bowl, combine granulated sugar, flour, and ground cinnamon for the topping. Add diced chilled butter and cut in with a pastry blender or fork until mixture forms coarse crumbs.
07 - Sprinkle streusel evenly over the batter. Bake for 40 to 45 minutes, until a toothpick inserted in the center comes out clean and the topping is golden brown.
08 - Transfer the pan to a wire rack and cool for at least 20 minutes before slicing and serving.