In a mixing bowl, use an electric mixer to beat the butter and sugar together until smooth and fluffy. Add the eggs and vanilla and mix until combined.
In a separate mixing bowl, mix together the dry ingredients (flour, baking powder, and salt.
Add half of the dry ingredients to the wet mixture. Mix until combined. Then add the remaining half and mix to combine.
Divide the dough in half and place each half in a separate bowl. Add green gel food coloring to one batch of dough and knead with your hands until you get the desired color. Repeat with the other half of the dough using the purple gel food coloring.
Roll each batch of dough into a bowl and wrap tightly in plastic wrap. Refrigerate for at least 1 hour.
Dust counter or cutting board and rolling pin lightly with flour.
Unwrap the purple dough and sprinkle with flour. Roll out into a rectangle and set aside. Repeat with the green cookie dough.
Use a spatula to lift the green dough and place it on top of the purple dough. (If dough is feeling too sticky, you can place it on parchment paper and refrigerate for a few minutes.)
Use your rolling pin to roll the two doughs together a bit. Then use a knife to trim the edges into straight lines, leaving you with a rectangle shape.
Carefully roll the dough into a log and wrap it with plastic wrap. Be sure to cover both ends of the cookie dough log with plastic wrap. Refrigerate for 1 hour or until the dough is firm again. (You can also place it in the freezer to make this step go more quickly.)
Preheat the oven to 325 degrees. Line a baking sheet with parchment paper.
Remove the dough from the refrigerator and remove the plastic wrap. Slice off the ends then slice the cookie dough log into 1/3 inch slices. (The ends don’t make pretty cookies but still taste good!)
Place 2 inches apart on the cookie sheet. Back for 13-15 minutes or until the cookies are set. (If you overcook them they will start to brown up so check on them after about 10 minutes.)
Allow to cool for 5 minutes before placing on a wire cooling rack to cool completely. Then enjoy!