1. Preheat oven to 350 degrees. Coat a 9-by-13-inch rimmed baking sheet with cooking spray, and line with parchment. Dust with flour, tapping out excess. Whisk together flour, baking powder, and salt in a bowl.

2. Cream butter and sugar with a mixer until pale and fluffy. Beat in zest if desired, and vanilla. Add eggs, 1 at time, beating well after each addition. Reduce speed to medium, and beat in flour mixture in 2 additions, alternating with milk.

3. Spread batter into prepared sheet. Bake until a tester inserted into the center comes out clean, 15 to 17 minutes. Let cool in sheet on a wire rack.