01 - Whisk together flour, baking powder, and salt in a medium bowl until well combined. 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 on low speed just until a cohesive dough forms. Avoid overmixing to prevent tough cookies.
04 - Separate dough into three equal portions. Mix one portion with cocoa powder until chocolate layer is uniform. Mix second portion with strawberry powder and optional food coloring until pink color is even. Keep third portion plain for vanilla layer.
05 - Roll each dough portion between two sheets of parchment paper into 10x6-inch rectangles. Refrigerate all three sheets for 20-30 minutes until firm enough to handle without sticking.
06 - Stack dough layers in order: chocolate on bottom, vanilla in middle, strawberry on top. Gently press layers together to ensure they adhere without distorting the rectangle shape.
07 - Starting from the long edge, tightly roll stacked dough into a log. Wrap completely 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 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 until edges are just lightly golden. Let cookies cool on baking sheet for 5 minutes, then transfer to wire rack to cool completely.