Preheat oven to 350F and line two baking sheets with parchment paper.
Whisk the gluten free all-purpose flour, cocoa powder, baking soda, and salt together in a bowl.
In a separate bowl, cream the softened butter and granulated sugar until fluffy. Add the egg and mix well. Alternately mix in the dry ingredients and buttermilk until a smooth, thick batter forms.
Drop mound-shaped spoonfuls of batter onto the prepared baking sheets, about 2 inches apart.
Bake for 10-12 minutes until the cakes spring back when touched. Let them cool completely on wire racks.
For the filling, beat the softened butter, powdered sugar, peppermint extract, and green food coloring until a fluffy green frosting forms.
Spread a generous amount of the mint filling onto the flat side of one chocolate cake, then top with a second cake to form a whoopie pie.