01 - In a medium bowl, whisk together the flour, baking powder, and salt until evenly combined. 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 vanilla extract until fully incorporated.
04 - Gradually add the flour mixture to the butter mixture, mixing on low speed until the dough just comes together. Do not overmix.
05 - Split 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 - Place each dough portion on parchment paper or plastic wrap and shape into a flat rectangle approximately 8 by 4 inches.
07 - Layer the rectangles on top of each other—chocolate on the bottom, vanilla in the center, and strawberry on top—pressing gently so they adhere. Wrap tightly and refrigerate for at least 45 minutes until firm.
08 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
09 - Using a sharp knife, slice the chilled dough log crosswise into ¼-inch thick rounds. Place 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. Rotate the sheets halfway through for even browning.
11 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.