01 - Whisk together flour, baking powder, and salt in a medium bowl. Set aside.
02 - Beat butter and sugar in a large bowl until light and fluffy. Add egg and vanilla; mix until fully combined.
03 - Gradually add flour mixture to wet ingredients, mixing just until dough forms. Avoid overmixing.
04 - Divide dough into three equal portions. Mix one portion with cocoa powder for chocolate layer. Mix second portion with strawberry powder (and food coloring if desired) for strawberry layer. Leave third portion plain for vanilla layer.
05 - Roll each dough portion between parchment paper sheets into equal rectangles approximately 10x6 inches. Chill each sheet for 20-30 minutes until firm.
06 - Layer chocolate dough on bottom, vanilla in the middle, and strawberry on top. Gently press layers together to adhere.
07 - Starting from long edge, roll stacked dough tightly into a log. Wrap in plastic wrap and chill for at least 1 hour until very firm.
08 - Preheat oven to 350°F. Line baking sheets with parchment paper.
09 - Slice chilled dough log into 1/4-inch rounds. Place cookies 2 inches apart on prepared baking sheets. Bake for 10-12 minutes until edges are lightly golden.
10 - Let cookies rest on baking sheet for 5 minutes, then transfer to wire rack to cool completely.