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