Cream Butter & Sugar:
Beat butter and sugar on medium-high speed 3–5 min until pale and fluffy. Add egg and vanilla; mix until combined.

Combine Dry Ingredients:
Whisk flour, baking powder, and salt. Gradually add to wet mixture on low speed, mixing just until a soft dough forms. Avoid overmixing.

Divide & Color:
Divide dough into 3 portions. Knead gel coloring into each portion until uniform (use gloves).

Roll, Stack & Roll Again:
Roll each color into a 9×7-inch rectangle, ¼-inch thick. Stack rectangles in desired order. Starting from a long edge, roll tightly into a log.

Wrap & Chill:
Wrap dough log in plastic wrap. Chill 1 hour (or up to 2 days).

Slice, Sprinkle & Bake:
Preheat oven to 350°F (175°C). Slice log into ¼-inch rounds. Press edges into sprinkles if desired. Place 2 inches apart on parchment-lined sheets. Bake 10–12 min until edges are set; centers slightly soft.

Cool:
Cool 5 min on sheets, then transfer to a wire rack to fully firm before serving.