01 - In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
02 - In a large bowl, beat the softened butter and sugar together using an electric mixer until light and fluffy, about 2 minutes.
03 - Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until fully blended.
04 - Gradually add the dry ingredient mixture to the wet ingredients, 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. Mix the cocoa powder and milk into a second portion until well blended. Leave the third portion plain for the vanilla layer.
06 - Place each dough portion onto parchment paper or plastic wrap and shape into a flat rectangle approximately 8 x 4 inches.
07 - Layer the rectangles on top of one another — chocolate on the bottom, vanilla in the middle, and strawberry on top — pressing gently so they adhere together.
08 - Wrap the stacked dough tightly and refrigerate for at least 45 minutes until firm enough to slice cleanly.
09 - Preheat the oven to 350°F. Line baking sheets with parchment paper.
10 - Using a sharp knife, cut the chilled dough log crosswise into ¼-inch thick slices. Arrange the slices 1 inch apart on the prepared baking sheets.
11 - Bake for 10 to 12 minutes, until the edges are just beginning to turn golden.
12 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.