Preheat oven to 375 degrees. Spray an 8x8 pan with non-stick spray.
Mix brown sugar, flour, and baking powder together in a stand mixer. Add lemon zest and salt.
Chop cold butter into cubes and add to the flour mix. Add in the egg and mix with the paddle attachment until blended. Batter will be crumbly.
In a small bowl, mix cornstarch and white sugar together, stir in blueberries and lemon juice.
Press half of the dough in the bottom of the pan. Add the blueberry mix and crumble the rest of the dough on top. Sprinkle with brown sugar.
Bake for 35 minutes and let cool in the fridge. Unlike a normal cookie, these bars hold together better in the fridge. Being cool also makes them easier to cut and serve.