In a saucepan, combine blueberries, lemon juice, lemon zest, granulated sugar, and a pinch of citric acid. Bring to a boil over medium heat, then reduce heat and simmer for about 10 minutes. Blend until smooth and strain through a sieve.

Cream together unsalted butter and powdered sugar until light and fluffy. Add egg yolk, vanilla extract, kosher salt, lemon juice, and zest, mixing until smooth. Gradually incorporate flour until just combined.

Divide the cookie dough into four portions. Color three portions with blueberry syrup to achieve different shades, and leave one portion plain. Mix until colors are evenly distributed.

Wrap each colored dough portion in plastic wrap and refrigerate for at least 2 hours.

Roll out each colored dough portion to about 1/4-inch thickness and layer them together. Use egg white as glue between colors and cut out desired shapes with cookie cutters.

Preheat the oven to 350°F (175°C). Bake cookies for 7 minutes, then turn off the oven and leave them inside for an additional 20 minutes.

Let the cookies cool on the tray for a few minutes before transferring them to a wire rack to cool completely. Store in an airtight container at room temperature for 4-5 days.