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, approximately 3-4 minutes. 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. Do not overmix.
04 - Divide dough into three equal portions. Mix one portion with cocoa powder for chocolate layer. Mix second portion with strawberry powder and optional food coloring. Leave third portion plain for vanilla layer.
05 - Roll each dough portion between two sheets of parchment paper into rectangles of equal size (approximately 10x6 inches). Refrigerate each sheet for 20-30 minutes until firm.
06 - Stack dough layers with chocolate on bottom, vanilla in middle, and strawberry on top. Press gently together. Starting from the long edge, tightly roll into a log. Wrap in plastic and refrigerate for at least 1 hour until very firm.
07 - Preheat oven to 350°F. Line baking sheets with parchment paper.
08 - 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.
09 - Let cookies rest on baking sheet for 5 minutes, then transfer to wire rack to cool completely.