Heat oven to 350°F (175°C) and prepare a baking sheet.
Beat butter and sugar for 2–3 minutes.
Add egg and vanilla.
Whisk flour, gelatin, baking powder, and salt.
Mix dry into wet until a soft dough forms.
Roll dough into balls, flatten, and top with sprinkles.
Bake for 8–10 minutes until edges set.
Cool 5 minutes on sheet, then rack.
