01 - In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
02 - In a large bowl, use an electric mixer to beat the softened butter and granulated sugar on medium speed 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 evenly distributed.
04 - Gradually add the flour mixture to the butter mixture, mixing on low speed until the dough just comes together. Avoid overmixing.
05 - Split the dough into three equal portions. To one portion, fold in the freeze-dried strawberry powder until uniformly pink. To a second portion, blend in the cocoa powder and milk until a smooth, even chocolate dough forms. Leave the third portion plain for the vanilla layer.
06 - Place each dough portion on a sheet of parchment paper or plastic wrap and shape into a flat rectangle roughly 8 by 4 inches, smoothing the surfaces evenly.
07 - Layer the rectangles directly on top of one another, starting with chocolate on the bottom, followed by vanilla, then strawberry on top. Gently press together so the layers adhere. Wrap the stacked block tightly and refrigerate for at least 45 minutes until very firm.
08 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
09 - Using a sharp knife, cut the chilled dough block crosswise into ¼-inch thick slices. Place each slice flat on the prepared baking sheets, spacing them about 1 inch apart.
10 - Bake for 10 to 12 minutes, rotating the sheets halfway through, until the edges are just beginning to turn a pale golden color. Do not overbake.
11 - Allow the cookies to rest on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely before serving or storing.