Do Ahead
1-Get the butter out of the refrigerator: Get the butter out of the refrigerator the night before or a few hours before you plan to bake to let it come to room temperature.
2-The blueberry reduction can be made a: The blueberry reduction can be made a few days ahead of time and stored in the refrigerator until ready to use.

Make the blueberry reduction
3-Stir together all of the ingredients for: Stir together all of the ingredients for the blueberry syrup in a medium saucepan over medium-high heat. Bring to a boil. Once boiling, remove from heat.
4-Add to a blender or use an: Add to a blender or use an immersion blender to blend the blueberries. Strain through a sieve back into the saucepan.
5-Return the saucepan to a low boil: Return the saucepan to a low boil and cook for 7-10 minutes, stirring occasionally. The blueberry sauce will thicken to coat the back of a spoon. Remove from heat and cool completely*.

Start the cookie dough
6-Combine the unsalted butter and powdered sugar: Combine the unsalted butter and powdered sugar in the bowl of a stand mixer fitted with the paddle attachment. Mix on low until fully combined.
7-Add the egg yolk and half of: Add the egg yolk and half of the egg white (saving the rest for later), vanilla, salt, lemon juice, lemon zest. Mix to combine.
8-Add in ⅓ of the flour at: Add in ⅓ of the flour at a time, mixing in between, just until combined.

Color the dough
9-Divide the dough evenly into 4 balls: Divide the dough evenly into 4 balls. Set aside one ball (this will be the plain lemon layer).
10-To the first ball, start by adding: To the first ball, start by adding two tablespoons of blueberry sauce and knead it in with your hands (I used gloves to prevent staining my hands). This will be the darkest layer. Add blue/purple food coloring to boost the color.
11-To the second ball, start by kneading: To the second ball, start by kneading in 1 ½ tablespoons of the blueberry sauce. Adjust with more or less sauce and/or food coloring according to how you want the color.
12-To the third ball, add ½ to: To the third ball, add ½ to 1 tablespoon of the blueberry sauce. Adjust with more or less sauce and/or food coloring according to how you want the color.
13-Wrap and chill each dough ball for: Wrap and chill each dough ball for at least 2 hours in the refrigerator.

Assemble the cookies
14-Roll out each dough to ⅛-inch thick: Roll out each dough to ⅛-inch thick between sheets of parchment or thin sheets of plastic.
15-Mix in a splash of water to: Mix in a splash of water to the reserved egg white. This will be the “glue” between the layers.
16-Starting with the plain lemon layer, brush: Starting with the plain lemon layer, brush a thin layer of “glue” on top. Add the next lightest blueberry dough layer on top. Brush the top of this layer with the “glue”. Add the next darkest blueberry dough layer on top of that. Brush with the “glue”, and finish by adding the darkest blueberry dough layer on top.
17-Gently press and smooth together the layers: Gently press and smooth together the layers to remove any air bubbles.
18-Punch out the cookies using the cookie: Punch out the cookies using the cookie cutter of your choice. I used a 2-inch flower cutter.
19-Place the cookies 1-inch apart on a: Place the cookies 1-inch apart on a baking tray lined with parchment paper or a silicon mat.

Bake the cookies
20-Preheat the oven to 350°F: Preheat the oven to 350°F.
21-Place the tray in the freezer to: Place the tray in the freezer to freeze solid while the oven preheats (ideally for about 10 minutes).
22-Bake for 7 minutes, remove the tray: Bake for 7 minutes, remove the tray and wait 5 minutes. Return the tray to the oven, turn it off and let it finish in the oven for 20 minutes. This method helps retain the vibrant colors.
23-Cool completely on a wire rack before: Cool completely on a wire rack before serving or storing.