01 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
02 - In a large bowl, beat butter and sugar together until light and fluffy. Add egg and vanilla; mix until combined.
03 - Gradually add the flour mixture to the wet ingredients, mixing just until a dough forms.
04 - Divide dough into three equal portions.
05 - Mix one portion with cocoa powder until fully incorporated.
06 - Mix one portion with freeze-dried strawberry powder (and a drop of pink food coloring, if desired) until even color is achieved.
07 - Leave the third portion plain for the vanilla layer.
08 - Roll each dough portion between two sheets of parchment paper into rectangles of equal size (about 10x6 inches). Chill each sheet for 20–30 minutes until firm.
09 - Carefully stack the dough layers: Place the chocolate layer on the bottom, top with the vanilla layer, then the strawberry layer. Gently press together.
10 - Starting from the long edge, tightly roll up the stacked dough into a log. Wrap the log in plastic wrap and chill for at least 1 hour until very firm.
11 - Preheat oven to 350°F. Line baking sheets with parchment paper.
12 - Slice chilled dough log into 1/4-inch rounds. Place cookies 2 inches apart on prepared baking sheets.
13 - Bake for 10–12 minutes or until the edges are just lightly golden. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.