Preheat the oven to 350°F (175°C) and line a baking sheet with parchment. Pro move: this keeps fingers and cookies happy.
Whisk together the flour, baking powder, and salt in a bowl. Dry ingredients get their own spa day before the dough arrives.
Cream the butter and sugar in a separate bowl until light and fluffy. This usually takes 2–3 minutes with an electric mixer. Scrape the sides so nothing hides up there.
Add the egg and vanilla (and lemon zest if you’re feeling fancy) and mix until smooth. Don’t overbeat—just until it comes together like a good playlist segue.
Gradually mix in the dry ingredients until a soft dough forms. It should be easy to roll, not rock-hard. If it’s sticky, chill for 15 minutes.
Roll dough into 1/2-inch balls or press gently with your hands. Place on the sheet about 2 inches apart.
Bake 8–10 minutes, until edges just begin to turn golden. The centers may look a little soft—that’s fine; they’ll firm up as they cool.
Cool on the sheet for 5 minutes, then transfer to a rack. Do not skip cooling, or your decorations will melt faster than your patience.