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. Add egg and vanilla extract; mix until fully incorporated.
03 - Gradually add the flour mixture to the wet ingredients, mixing just until a cohesive dough forms. Avoid overmixing.
04 - Split dough into three equal portions. Mix one portion with cocoa powder for chocolate layer. Mix second portion with strawberry powder and food coloring if desired. Leave third portion plain for vanilla layer.
05 - Roll each dough portion between two sheets of parchment paper into 10x6-inch rectangles. Refrigerate each sheet for 20–30 minutes until firm.
06 - Layer dough sheets with chocolate on bottom, vanilla in middle, and strawberry on top. Press gently to adhere. Roll tightly from long edge into a log. Wrap in plastic wrap and refrigerate for at least 1 hour until very firm.
07 - Preheat oven to 350°F. Line baking sheets with parchment paper.
08 - Cut chilled dough log into 1/4-inch rounds. Arrange 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.