Add 600 grams fresh or frozen blueberries, 1/2 cup granulated sugar, and 2 Tablespoons cornstarch to a medium saucepan and stir. Heat the saucepan over medium-high heat, stirring often until it forms a thick sauce and comes to a boil, about 10-15 minutes. Stir in 1 Tablespoon lemon juice and 1 teaspoon lemon zest. Remove the pot from the heat and set aside to cool slightly while you prep the other components of the bars.
Cream Cheese Swirl
Add 4 ounces softened cream cheese, and 1/4 cup sugar to a medium bowl. Use a hand mixer (or spatula) to cream them together. Beat in 1 large egg and 1/2 teaspoon vanilla extract until smooth. Set aside.
Cookie Crust
Prepare a 9×13" baking pan by lining it with parchment. Turn oven on to 350°F.
Whisk together 3 cups all-purpose flour, 1 1/2 teaspoons baking powder, and 1/2 teaspoon salt in a medium bowl. Set aside.
Add 1 cup softened unsalted butter and 1 1/2 cups granulated sugar to the bowl of your stand mixer (or a large bowl). Cream the butter and sugar together at medium-high speed. Add 2 large eggs and 1 teaspoon lemon zest and beat the mixture again at medium-high speed until it's light and fluffy, scraping down the sides of the bowl as needed.
Add the flour mixture to the butter mixture. Mix at low speed until the flour is completely incorporated and a dough forms.
Press 2/3 to 3/4 of the cookie crust dough evenly into the prepared baking pan. Spread the blueberry filling over the dough. Spread the cream cheese swirl evenly over the blueberry filling. Gently swirl the two layers together, if desired.
Drop small pieces of the remaining dough evenly over the top of the filling. Bake for 40-50 minutes until the top of the crust is light brown. Cool for at least 20 minutes before cutting.