Cream butter and sugar until light and fluffy. Add egg and vanilla; beat until combined.

Whisk flour, baking powder, and salt in a separate bowl. Gradually mix into the wet ingredients to form dough.

Divide dough into 2 equal portions.

Tint one red, one green (or any colors you love).

Knead color in gently to avoid overworking.

Roll out each colored dough between two sheets of parchment into equal rectangles (about ¼-inch thick).

Stack one sheet over the other and gently roll into a tight log. Wrap in plastic wrap.

Chill for at least 2 hours (or overnight) until firm.

Slice into ¼-inch rounds. Roll edges in sprinkles if desired.

Bake at 350°F (175°C) for 8–10 minutes, or until set but not browned.

Cool on baking sheet, then transfer to wire rack.