Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
Whisk flour, baking powder, baking soda, and salt in a bowl.
In another bowl, beat softened butter with both sugars for 2–3 minutes until fluffy.
Add egg and vanilla, mixing until smooth.
Gradually stir in dry ingredients until just combined — don’t overmix.
Scoop tablespoon-sized dough balls, place 2 inches apart on trays, and flatten tops slightly.
Bake 10–12 minutes, until edges turn golden and centers are still soft.
Cool on tray for a few minutes, then transfer to a rack to firm up.