Preheat the oven to 350˚F (175˚C). Line two to four baking trays with parchment paper.
In a large bowl, using an electric whisk or stand mixer, beat the butter and sugar until light and fluffy. This will take 2 to 3 minutes.
Add the egg, flour and vanilla. Beat until you have a smooth dough. Divide the dough in half. Add green food coloring to one half. Mix this in with your hands until the color is evenly distributed throughout.
Roll the green dough into 12 equal-sized balls, then flatten them down.
Take a butter knife and create the crosshatch pattern on top to create the shell. Set them aside.
Wash your hands here to make sure there is no green food coloring on them.
Roll the rest of the plain dough into 48 small balls and 12 medium balls. Squash them down to flatten slightly.
On your lined baking trays, place four small balls in a square shape.
At the top of the square, add the medium ball.
Place the green shell on top. Carefully flatten it slightly to stick the dough together.
Repeat this process with the remaining dough.
Using a skewer, poke two small holes in the heads for eyes.
Once all turtles are assembled, bake them in the oven for 10 to 12 minutes, or until the edges start to turn golden brown.
Leave to cool completely at room temperature before serving.