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 combined.
03 - Gradually incorporate the flour mixture into the wet ingredients, mixing just until dough forms. Avoid overmixing to prevent tough cookies.
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 layer.
05 - Roll each dough portion between parchment paper sheets into 10x6 inch rectangles. Refrigerate all sheets for 20-30 minutes until firm enough to handle without sticking.
06 - Stack dough layers in order: chocolate on bottom, vanilla in center, strawberry on top. Press gently to adhere layers without compressing.
07 - Roll stacked dough starting from long edge into tight log. Wrap completely in plastic wrap and refrigerate for minimum 1 hour until very firm.
08 - Preheat oven to 350°F. Line baking sheets with parchment paper and set aside.
09 - Remove chilled log from wrapper. Cut into 1/4-inch thick rounds using sharp knife. Place cookies 2 inches apart on prepared baking sheets.
10 - Bake for 10-12 minutes until edges are lightly golden. Allow cookies to rest on baking sheet for 5 minutes before transferring to wire rack for complete cooling.