Beat at medium speed with an electric mixer until creamy
Gradually add sugar beating well
Stir in vanilla extract
Combine flour and cornstarch gradually add to butter mixture beating at lowest speed after each addition
Press into a 15 x 10 x 1-inch jellyroll pan
Bake at 350 for 15 minutes
Reduce heat to 325 and bake 20 additional minutes or until top is lightly browned Cool in pan 5 minutes on a wire rack cut shortbread into 1 inch squares
Store in an airtight container
at room temperature up to 1 week or freeze up to 3 months