Line a 15x10x1-inch baking pan with foil, allowing foil to extend over short edges of pan; grease foil and set aside. In a medium bowl, combine flour, baking powder and salt; set aside. In a large mixing bowl, beat 1 cup butter with an electric mixer on medium to high for 30 seconds. Add 1-1/4 cups granulated sugar; beat until combined. Beat in egg, lemon peel, lemon juice and vanilla until combined. Beat in as much of the flour mixture as you can with the mixer. Using a wooden spoon, stir in any remaining flour mixture. Spread dough into prepared pan.