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 granulated sugar with an electric mixer until pale, 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 incorporated.
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. Add the freeze-dried strawberry powder (or food coloring and extract) to one portion and mix until evenly colored. Add the cocoa powder and milk to a second portion and blend thoroughly. 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.
07 - Stack the rectangles on top of one another — chocolate on the bottom, vanilla in the middle, and strawberry on top. Gently press the layers together so they adhere.
08 - Wrap the stacked dough tightly in plastic wrap and refrigerate for at least 45 minutes until firm.
09 - Preheat the oven to 350°F. Line baking sheets with parchment paper.
10 - Using a sharp knife, slice the chilled dough log crosswise into ¼-inch thick slices. Place the cookies 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 - Let the cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.