In a medium bowl, whisk together the flour, baking powder, and salt until well combined.

In a large bowl, use an electric mixer to beat the butter and sugar on medium-high speed until well-combined and smooth, about 2 minutes. Scrape down the sides and bottom of the bowl. Add the egg, peppermint extract, and vanilla extract and beat until combined, then scrape down the bowl again. On low speed, gradually add the flour mixture and mix until just incorporated.

Divide the dough in half (it’s ok if it’s not exact) and place one half in a small bowl. Starting with ¼ teaspoon, add green food coloring and mix with a spatula until evenly colored. Add more coloring if desired.

Into the remaining plain dough, add cocoa powder and beat on low speed until just combined. Place each half of dough on a sheet of plastic wrap. Wrap and shape into a small rectangle. Refrigerate until firm but still pliable, about 1 hour.

Place a sheet of parchment paper on top of a silicone-lined baking mat. Place one half of the dough on top. Cover with plastic wrap. Roll dough into an 8x14-inch large rectangle about 1/4-inch thick. Repeat with remaining dough. Place both doughs, still covered in plastic, in the fridge or freezer until firm but pliable, about 10 minutes.

Once chilled, remove the plastic wrap from the green dough. Place dough down on a fresh sheet of parchment. Carefully peel away the parchment attached to the dough. Repeat the procedure to place the chocolate dough on top of the green dough, lining up as closely as possible. Press the dough together to ensure there are no gaps between layers. Trim the dough layers to create an even rectangle if needed.

Using the parchment paper under the green dough as a guide, slowly roll the long side of the dough into a tight log, making sure there are no gaps. Wrap log in parchment or plastic wrap and freeze for 1 hour or until firm.

Line large baking sheets with parchment paper.