In a small bowl, whisk together flour, baking powder and salt; set aside.
Beat butter and sugar on medium-high speed for 2 minutes or until smooth. Beat in egg, zest and juice until fluffy, 1 minute. Gradually beat in flour mixture on low, just until blended. Add poppy seeds; stir to combine. Divide dough in half. Flatten each into 6-inch square; wrap in plastic. Refrigerate 2 hours or overnight. Let stand at room temperature 15 minutes before rolling out dough.
Heat oven to 350 degrees F. Coat 2 baking sheets with nonstick cooking spray. Lightly flour dough. Roll half of dough on lightly floured surface to 1/4-inch thickness. For variety, use 2 cutters, one about 2 inches and the other 2 1/2 inches. Reroll scraps and repeat. Place cookies, 1 inch apart, on baking sheet. Repeat with other half of dough.
Bake at 350 degrees F for 10 to 13 minutes or until golden around edges. Remove to rack to cool completely.