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 combined.
03 - Gradually add the flour mixture to the wet ingredients, mixing just until a dough forms. Be careful not to overmix.
04 - Divide dough into three equal portions. Mix one portion with cocoa powder for the chocolate layer. Mix another portion with strawberry powder and optional food coloring. Leave the third portion plain for vanilla.
05 - Roll each dough portion between two sheets of parchment paper into rectangles of equal size (approximately 10x6 inches). Chill each sheet for 20-30 minutes until firm.
06 - Stack the dough layers with chocolate on the bottom, vanilla in the middle, and strawberry 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 in plastic wrap and chill for at least 1 hour until very firm.
08 - Preheat oven to 350°F. Line baking sheets with parchment paper.
09 - 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.
10 - Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before storing.