Preheat your oven to 350°F (175°C) to ensure it’s hot enough for even baking.

Grease an 8x8-inch square baking pan lightly and line it with parchment paper.

In a mixing bowl, combine softened butter and sugar, beating until creamy. Gradually add gluten-free flour and cornstarch, mixing until crumbly.

Gently toss blueberries with cornstarch and lemon juice, then spread this mixture over the crust.

Mix the remaining crust mixture with butter until crumbly, then sprinkle over the blueberries.

Bake for 60 minutes until golden brown. A toothpick should come out mostly clean.

Cool completely in the pan on a wire rack, then cut into squares.