Step 1: Preheat your oven to 350°F (175°C).
Step 2: Grease your 9x9 inch baking dish lightly with butter or line it with parchment paper.
Step 3: In a mixing bowl, combine flour, powdered sugar, and a dash of salt. Add cold butter and mix until coarse crumbs. Press into the bottom of the baking dish.
Step 4: In another bowl, mix blueberries with sugar and lemon juice. Optionally add cornstarch.
Step 5: Spread the blueberry mixture evenly over the crust.
Step 6: In the same bowl, combine flour, powdered sugar, and remaining cold butter for streusel. Mix until crumbly and sprinkle over the blueberries.
Step 7: Bake for 30-35 minutes until topping is golden and blueberries are bubbling.
Step 8: Cool completely in the dish, then cut into squares and serve.