Position rack in center of oven and preheat to 350°F. Grease a 9- by 13-inch baking pan with butter (or spray with cooking spray); set aside.
In a medium bowl, whisk together flour, baking powder, salt, and lemon zest. In a large bowl, use an electric mixer to stir butter until creamy. Blend in sugar and beat on medium-high speed for 3 minutes or until light and fluffy. Mix in the eggs, one at a time, and add the lemon juice, vanilla extract, and lemon extract, if using. Blend until just smooth.
Mix in the flour mixture until well combined. Spread in an even layer in the prepared pan. Sprinkle the granulated sugar and sparkling sugar evenly over the top of the batter.
Bake for 22 to 28 minutes or until a toothpick inserted in the center comes out clean (do not overbake). Cool in the pan on a wire rack. Cut into 20 bars.
If you don't have any sparkling sugar on hand, you may top the unbaked bars with 3 tablespoons of granulated sugar.