1. Combine blueberries, lemon juice, lemon zest, sugar, and citric acid in a saucepan. Bring to a boil then blend until smooth and strain.

2. Return to low boil and cook 7 to 10 minutes until thickened. Cool completely.

3. Beat butter and powdered sugar until smooth. Add egg yolk, half egg white, vanilla, salt, lemon juice, and zest.

4. Gradually add sifted flour in thirds and mix until dough just comes together.

5. Divide dough into 4 balls. Knead increasing amounts of blueberry syrup into 3 of them for a gradient. Add food coloring if desired.

6. Wrap each ball and refrigerate at least 2 hours.

7. Roll each dough ball to 1/8 inch thickness between parchment sheets.

8. Layer doughs from plain lemon to darkest blueberry using egg white glue between each layer.

9. Cut out shapes with a 2 inch cookie cutter and place on lined baking sheet.

10. Freeze assembled cookies for 10 minutes.

11. Bake at 350°F for 7 minutes. Remove and rest 5 minutes at room temperature.

12. Return to oven, turn oven off, and finish in residual heat for 20 minutes.

13. Cool completely on a wire rack before serving.