01 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until evenly distributed. Set aside.
02 - In a large bowl, using an electric mixer, beat the softened unsalted butter and granulated sugar on medium speed until 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 combined.
04 - Gradually add the reserved dry ingredient mixture to the wet mixture, mixing on low speed until the dough just comes together. Avoid overmixing.
05 - Divide the dough into three equal portions, each approximately one-third of the total weight.
06 - Add the freeze-dried strawberry powder (or food coloring and strawberry extract) to one portion of dough. Knead gently until the color is uniform throughout.
07 - Add the unsweetened cocoa powder and milk to the second portion of dough. Mix thoroughly until no dry streaks remain and the color is even.
08 - Leave the third portion of dough plain. This will serve as the vanilla layer.
09 - Place each dough portion on a sheet of parchment paper or plastic wrap and shape into a flat rectangle approximately 8 x 4 inches.
10 - Stack the three rectangles on top of each other—chocolate on the bottom, vanilla in the middle, and strawberry on top. Gently press the layers together so they adhere.
11 - Wrap the stacked dough tightly in parchment paper or plastic wrap and refrigerate for at least 45 minutes until the dough is firm enough to slice cleanly.
12 - Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
13 - Using a sharp knife, slice the chilled dough crosswise into ¼-inch thick slices. Place each slice 1 inch apart on the prepared baking sheets.
14 - Bake for 10 to 12 minutes, until the edges are just beginning to turn golden. Rotate the baking sheet halfway through for even browning.
15 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before serving or storing.