01 - Whisk together flour, baking powder, and salt in a medium bowl until thoroughly combined. Set aside for later use.
02 - Beat softened butter and granulated sugar in a large bowl until mixture becomes 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 on low speed just until a cohesive dough forms. Be careful not to overmix.
04 - Separate dough into three equal portions. For the chocolate layer, mix one portion with cocoa powder until completely integrated. For the strawberry layer, combine another portion with strawberry powder and optional pink food coloring until evenly colored. Leave the remaining portion plain for vanilla.
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 to the touch.
06 - Carefully place the chocolate layer on the bottom, followed by the vanilla layer, then the strawberry layer on top. Gently press layers together to ensure they adhere.
07 - Starting from the long edge, tightly roll the stacked dough into a log. Wrap securely in plastic wrap and refrigerate for at least 1 hour until very firm and easy to slice.
08 - Preheat oven to 350°F. Line baking sheets with parchment paper and set aside.
09 - Slice the chilled dough log into 1/4-inch rounds using a sharp knife. Place cookies 2 inches apart on prepared baking sheets to allow for spreading.
10 - Bake for 10-12 minutes or until edges are just lightly golden. Allow cookies to cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.