Preheat oven to 350°F. Lightly spray a 9x13-inch baking dish with cooking spray and line it with parchment paper, leaving an overhang on two opposite sides to lift the bars out later.

In a large bowl, whisk together 2 1/2 cups all-purpose flour, 1/2 cup confectioners' sugar, and 3/4 teaspoon salt.

Add 1 cup cold salted butter (cubed) to the dry mixture. Use a pastry blender or two forks to cut the butter in until the mixture is crumbly and resembles coarse crumbs.

Press the crumb mixture evenly into the bottom of the prepared baking dish, compacting it firmly.

Bake the crust for 25 minutes, or until the edges are golden. Remove from the oven.

While the crust bakes, whisk the filling in a large bowl: 1 1/2 cups granulated sugar, 1 cup whole buttermilk, 1/2 cup melted salted butter, 3 large eggs, 2 teaspoons lemon zest, 3 tablespoons fresh lemon juice, 2 tablespoons plain yellow cornmeal, 1 tablespoon all-purpose flour, and 1 1/2 teaspoons vanilla extract until smooth and fully combined.

Let the crust cool for about 5 minutes so it is not piping hot, then pour the filling evenly over the crust.

Return the pan to the oven and bake for 30 minutes, or until the filling is set (the edges should be set and the center should no longer jiggle).

Remove from the oven and cool completely in the pan. Use the parchment overhang to lift the bars out of the pan, place on a cutting board, and cut into squares.

Store the bars covered in the refrigerator. Optional: sift additional confectioners' sugar over the bars before serving.