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 together using an electric mixer 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 dry ingredient mixture to the wet ingredients, mixing until just combined. Avoid overmixing.
05 - Split the dough into three equal portions. Add the freeze-dried strawberry powder to one portion and mix until evenly colored. Add the cocoa powder and milk to a second portion and mix until well blended. 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 x 4 inches. Layer the rectangles on top of each other—chocolate on the bottom, vanilla in the middle, and strawberry on top—gently pressing together so they adhere.
07 - Wrap the stacked dough tightly and refrigerate for at least 45 minutes until firm.
08 - Preheat the oven to 350°F. Line baking sheets with parchment paper.
09 - Using a sharp knife, slice the chilled dough crosswise into ¼-inch thick slices. Arrange the cookies 1 inch apart on the prepared baking sheets.
10 - Bake for 10 to 12 minutes, until the edges are just turning golden.
11 - Allow the cookies to cool for 5 minutes on the baking sheet, then transfer to a wire rack to cool completely.