01 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
02 - In a large bowl, beat the softened butter and granulated sugar with an electric mixer until pale, light, and fluffy, approximately 2 minutes.
03 - Add the eggs one at a time, beating well after each addition. Mix in the pure vanilla extract until fully incorporated.
04 - Gradually add the flour mixture to the butter mixture, mixing on low speed until just combined. Avoid overmixing.
05 - Divide the dough into three equal portions. Knead the freeze-dried strawberry powder into one portion until evenly colored. Blend the cocoa powder and milk into a second portion until uniform. Leave the third portion plain for the vanilla layer.
06 - On parchment paper or plastic wrap, press each dough portion into a flat rectangle measuring approximately 8 x 4 inches. Stack the rectangles—chocolate on the bottom, vanilla in the middle, and strawberry on top—pressing gently to adhere the layers.
07 - Wrap the stacked dough tightly in plastic wrap and refrigerate for at least 45 minutes until very firm.
08 - Preheat the oven to 350°F. Line baking sheets with parchment paper.
09 - Using a sharp knife, slice the chilled dough log crosswise into ¼-inch thick slices. Place the cookies 1 inch apart on the prepared baking sheets.
10 - Bake for 10 to 12 minutes, until the edges are just beginning to turn golden. Do not overbake.
11 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.