In a large bowl, mix melted butter, granulated sugar, and brown sugar with a silicone spatula until smooth.
Add the egg and vanilla extract to the mixture and stir until fully combined.
Add the flour, baking powder, baking soda, and salt to the wet ingredients and mix until mostly combined.
Fold in the Christmas sprinkles gently.
Cover the dough with plastic wrap and refrigerate for 30 minutes or up to 2 days.
Preheat the oven to 350°F (177°C) and line two baking sheets with parchment paper.
Use a cookie scoop to portion 2-3 tablespoon-sized dough balls on the prepared baking trays, spaced 3-4 inches apart.
Bake one sheet at a time for 9 to 11 minutes, until the edges are lightly golden and centers are mostly set.
Optionally, shape freshly baked warm cookies by gently rotating a large round cutter or mug on top to create perfect circles.
Top the cookies with additional sprinkles using your hands or tweezers.
Cool cookies on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before serving.