Prepare the blueberry jam by heating berries and sugar in the microwave until they burst, then chill the mixture.

Mix the cold butter, brown sugar, and flour in a small bowl with a fork until it forms coarse crumbs, then keep it cold in the fridge.

Whisk the flour, cornstarch, baking powder, baking soda, and salt together in a medium bowl and set aside.

Cream the softened butter, brown sugar, and lemon zest in a large bowl until the mixture is light and fluffy.

Beat in the egg, vanilla extract, and fresh lemon juice until the wet ingredients are fully combined.

Gradually stir the dry flour mixture into the wet ingredients on low speed until just combined, being careful not to overmix.

Gently fold the fresh blueberries into the dough using a spatula to keep them from breaking and turning the dough purple.

Scoop the dough onto the baking sheets and create a small well in the center of each ball.

Fill the centers with a small spoonful of the chilled jam and sprinkle the cold streusel crumble over the top.

Bake until the edges are golden brown and the topping is crisp, then let them rest on the pan before moving to a wire rack.