Mix Dry Ingredients: In a bowl, combine the flour, baking powder, baking soda, and salt.
Beat Butter and Sugar: In another bowl, beat the butter and sugar for 4 minutes until light and fluffy.
Add Wet Ingredients: Mix in the egg, lemon zest, lemon juice, and vanilla extract until combined.
Combine and Mix: Add the dry ingredients and poppy seeds to the wet mixture. Mix until just combined.
Scoop and Chill: Scoop the dough into balls (about ¼ cup each) and place on a baking sheet. Chill in the fridge for 15 minutes before baking. You can also freeze the dough balls for up to a month.
Bake: Preheat the oven to 350°F. Bake for about 20 minutes until the cookies are puffed and golden around the edges. For smaller cookies, reduce baking time. If using a convection oven, bake at 345°F for a shorter time.
Cool: Let the cookies cool completely before frosting.