Neapolitan Checkerboard Shortbread Cookies

Place the freeze-dried strawberries in a food processor and pulse until they form a fine powder. Sift the powder to remove any large seeds. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the softened butter and powdered sugar together on medium speed until smooth and light, about 2 minutes. Do not overbeat, as incorporating too much air will cause the cookies to spread during baking.

Add the vanilla extract and salt to the butter mixture, mixing briefly to combine. Gradually add the all-purpose flour on low speed, mixing just until the dough comes together in soft clumps.

Turn the dough out onto a work surface and gather it together. Weigh the total mass of the dough, then divide it evenly into three equal portions using your kitchen scale. Each portion should weigh approximately 340 grams.

Leave the first portion plain to serve as the vanilla layer. Take the second portion and knead in the cocoa powder until the color is uniform. If the chocolate dough feels crumbly, knead in the whole milk. Take the third portion and knead in the sifted strawberry powder until fully incorporated and evenly pink.

Shape each of the three colored doughs into a rectangular block measuring exactly 1.5 centimeters thick, 13.5 centimeters wide, and 15 centimeters long. Use a ruler and a bench scraper to make the edges as straight and square as possible.

Wrap each rectangular block tightly in plastic wrap and chill in the refrigerator until firm enough to slice cleanly without squishing, about 30 minutes.

Remove the dough blocks from the refrigerator. Using a sharp knife and ruler, trim any uneven edges. Cut each rectangle lengthwise into exactly nine strips that are 1.5 centimeters wide. You will now have 9 vanilla strips, 9 chocolate strips, and 9 strawberry strips.

Assemble three separate logs. For the first log, place a vanilla strip, a chocolate strip, and a strawberry strip side by side. Brush the facing sides with a very light coating of egg white to glue them together. Brush the top of this layer with egg white, then stack a second layer of three strips on top, offsetting the colors. Repeat for the third layer to form a 3x3 checkerboard. Press gently but firmly to secure the grid. Repeat this process to build two more logs from the remaining strips.

Wrap each assembled checkerboard log tightly in plastic wrap. Refrigerate until completely firm, at least 2 hours or preferably overnight. This resting period is critical for preventing the cookies from bleeding colors or losing their square shape when sliced.

Preheat your oven to 175C (350F). Line two baking sheets with parchment paper. Remove one log from the refrigerator, unwrap it, and use a sharp knife to slice it into cookies about 1 centimeter thick. Rotate the log slightly after every few slices to prevent the bottom from flattening.

Place the slices on the prepared baking sheets, leaving about 3 centimeters of space between them. Bake for 12 to 14 minutes, or until the vanilla squares are set and just beginning to turn a very pale golden on the bottom edges. Do not let them brown.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.