Step 1Cream 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 2Press 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.
Step 1Heat milk, margarine, sugar and syrup in saucepan over medium heat, stirring often, until mixture begins to come away from sides of pan, 8 to 10 minutes. Pour over cooked shortbread (still in its pan) and cool.
Step 2Melt chocolate chips in double boiler set over, but not touching, simmering water, or in microwave, and spread in layer over set caramel. Cool. Cut into fingers or squares.