01 - Whisk together flour, baking powder, and salt in a medium bowl. Set aside for later use.
02 - Beat butter and sugar in a large bowl until light and fluffy. Add egg and vanilla, mixing until fully combined.
03 - Gradually incorporate flour mixture into wet ingredients, mixing just until dough comes together. 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.
05 - Roll each dough portion between parchment sheets into 10x6-inch rectangles. Refrigerate for 20-30 minutes until firm.
06 - Layer dough sheets in order: chocolate on bottom, vanilla in middle, strawberry on top. Press gently to adhere layers.
07 - Starting from long edge, roll stacked dough tightly into a log. Wrap in plastic and refrigerate at least 1 hour until very firm.
08 - Heat oven to 350°F. Line baking sheets with parchment paper.
09 - Cut chilled log into 1/4-inch rounds. Arrange cookies 2 inches apart on prepared baking sheets.
10 - Bake for 10-12 minutes until edges are lightly golden. Cool on sheets for 5 minutes, then transfer to wire rack.