Prep dry ingredients: In a bowl, whisk together flour, baking powder, and salt. Set aside.

Cream butter and sugar: In a large bowl, beat butter and sugar 1–2 minutes until pale and fluffy. Mix in egg and vanilla until combined.

Form dough: Add flour mixture in two additions, mixing just until combined. Scrape down bowl as needed.

Divide and color: Split dough into three equal portions. Tint one red, one green, and leave one plain. Wrap each portion in plastic wrap and flatten into squares. Refrigerate 1 hour.

Roll dough: Roll each color between parchment into 10‑inch squares. Chill again 30 minutes to firm up.

Assemble layers: Stack doughs with red on bottom, plain in the middle, and green on top, carefully removing parchment between each. Trim edges for clean sides.

Form log: Starting from one side, roll tightly into a log. Press seam to seal, wrap in plastic, and refrigerate 1 hour or until firm.

Preheat oven: Set to 350°F (177°C) and line baking sheets with parchment paper.

Slice and bake: Cut log into ¼‑inch rounds with a sharp serrated knife. Arrange 2 inches apart on sheets. Bake 12–15 minutes until set.

Cool: Let cookies rest 5 minutes on the pan before transferring to a wire rack to cool completely.