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