Heat oven to 350°F (175°C) and line two baking sheets with parchment. Whisk melted butter and sugar until glossy, then whisk in egg and vanilla until smooth.

Stir in flour, baking powder, and salt until just combined, then fold in 1/2 cup of the sprinkles.

Scoop tablespoon-sized dough balls, roll between damp palms, then coat heavily in extra sprinkles and place 2 inches apart on sheets.

Bake 10 to 12 minutes until edges are just golden. Cool on the sheet for 5 minutes, then move to a wire rack.