Step 1 Cream butter gently in electric mixer using dough hook until light, 3 to 4 minutes. Add powdered sugar and continue beating a few more minutes. Slowly add flour and cornmeal and continue to mix with dough hook until dry ingredients are incorporated, 4 to 5 minutes. Dough will be dry.
Step 2 Press dough into 13x9-inch baking pan, and pierce repeatedly across bottom with fork to prevent it from rising. (If making plain shortbread, mark dividing lines between cookies same way).
Step 3Bake at 275 degrees 1 hour. Turn oven off and open door to allow some heat to escape. Leave shortbread in oven to continue to dry out another 30 minutes. Remove from oven and cool but do not remove from pan.