1. In a medium bowl, whisk together the flour, baking powder, and salt.

2. In a large bowl, beat the softened butter and granulated sugar until light and fluffy.

3. Add the egg and vanilla extract, then mix until smooth and fully combined.

4. Gradually add the dry ingredients to the wet ingredients and mix just until a soft dough forms.

5. Divide the dough evenly into two portions. Tint one portion red and the other green, mixing each until the color is evenly blended.

6. Roll each dough portion between sheets of parchment paper into matching rectangles about 1/4 inch thick.

7. Chill the dough briefly if needed, then stack one rectangle over the other and press gently to seal them together.

8. Starting from the long side, roll the layered dough into a tight log. Roll the outside in sprinkles if using.

9. Wrap the dough log and chill for at least 1 hour, or until very firm.

10. Preheat the oven to 350°F and line a baking sheet with parchment paper.

11. Slice the chilled dough into 1/4-inch rounds and place them on the prepared baking sheet.

12. Bake for 9 to 11 minutes, until the edges are set but not deeply browned.

13. Let the cookies cool on the baking sheet for 3 minutes, then transfer to a wire rack to cool completely.