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 combined.
03 - Gradually incorporate flour mixture into wet ingredients, mixing just until dough comes together. Avoid overmixing.
04 - Separate dough into three equal portions. Mix cocoa powder into one portion for chocolate layer. Blend strawberry powder (and food coloring if using) into second portion. Leave third portion plain for vanilla.
05 - Roll each dough portion between parchment sheets into equal rectangles approximately 10x6 inches. Refrigerate all layers for 20-30 minutes until firm.
06 - Layer dough sheets in order: chocolate on bottom, vanilla in center, strawberry on top. Press gently to adhere layers together.
07 - Roll stacked dough tightly from long edge into a log. Wrap in plastic and refrigerate for minimum 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 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 finish cooling.