Spiral Sugar Cookies

Line 2 rimless baking sheets with parchment paper or grease. Set aside.

In large bowl, beat butter with sugar until fluffy; beat in egg and vanilla.

In separate bowl, whisk flour with baking powder. Add to butter mixture in 2 additions, stirring to form soft dough.

Divide dough in half. Mix rainbow sprinkles into 1 half; using tiny dab of food colouring on end of toothpick, tint remaining half. Press each half into rectangle; wrap in plastic wrap and refrigerate until chilled, at least 1 hour. (Make-ahead: Refrigerate for up to 3 days or place in airtight container and freeze for up to 2 weeks.)

Remove plastic wrap. Between waxed paper, roll each rectangle into 9- x 8-inch (23 x 20 cm) rectangle; remove top papers. Removing bottom paper, stack 1 rectangle on top of the other. Using bottom paper as support and starting at 1 long edge, roll up tightly; press seam to seal. Wrap in plastic wrap; refrigerate until firm, at least 2 hours.

Cut log into 1/4-inch (5 mm) thick slices. Place, 2 inches (5 cm) apart, on prepared pans. Bake in top and bottom thirds of 375°F (190°C) oven, switching and rotating pans halfway through, until light golden, about 8 minutes. Transfer to rack; let cool completely. (Make-ahead: Layer between waxed paper in airtight container and store at room temperature for up to 5 days or freeze for up to 1 month.)