Beat butter, sugar, and vanilla until creamy.
Mix in egg until combined.
Add flour and baking powder, mix until smooth.
Split dough, tint one half red.
Wrap and chill for 30 minutes.
Roll into rectangles and layer doughs.
Roll tightly, chill for 2 hours.
Roll logs in sprinkles, slice.
Bake 9–11 minutes, cool on rack.
