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 with an electric mixer until light and fluffy, about 2 minutes.
03 - Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Mix in the vanilla extract.
04 - Gradually add the flour mixture to the butter mixture, mixing on low speed until just combined. Do not overmix.
05 - Divide the dough into three equal portions, approximately 320 grams each.
06 - Add the freeze-dried strawberry powder (or food coloring and strawberry extract) to one portion of dough. Mix until evenly colored throughout.
07 - Add the cocoa powder and milk to a second portion of dough. Mix until well blended and uniform in color.
08 - Leave the third portion plain; this is the vanilla layer.
09 - Place each dough portion on parchment paper or plastic wrap and shape into a flat rectangle approximately 8 x 4 inches.
10 - Layer the rectangles on top of each other—chocolate on the bottom, vanilla in the middle, and strawberry on top. Gently press together so they adhere.
11 - Wrap the stacked dough tightly in plastic wrap and refrigerate for at least 45 minutes until firm.
12 - Preheat oven to 350°F. Line baking sheets with parchment paper.
13 - Using a sharp knife, slice the chilled dough log crosswise into ¼-inch thick slices. Arrange cookies 1 inch apart on the prepared baking sheets.
14 - Bake for 10 to 12 minutes, until the edges are just beginning to turn golden.
15 - Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.