Line an 8x8-inch or 9x9-inch baking pan with parchment paper.
Whisk together the melted butter, oil and brown sugar in a large bowl, then whisk in the egg, pumpkin, and buttermilk.
Whisk in the baking powder, baking soda, salt and cinnamon.
Stir in the gluten free flour until no streaks of flour remain. Fold in the chocolate chips.
Pour batter into prepared pan and smooth the surface with an offset spatula. Preheat the oven to 325℉ and rest the batter while the oven heats.
Bake the for 45-55 minutes until a skewer comes out clean, then transfer it to a wire rack to cool.
Notes
Rest the batter. My homemade gf flour blend contains rice flour, and I always make sure to rest the batter for up to 20 minutes before baking. This helps the flour blend to hydrate - soak up some of the liquid - to avoid any grittiness later. I advise doing this for any type of gluten-free flour or blend you use.
Make cupcakes! You can make cute little pumpkin chocolate chip cake cupcakes by dividing the batter between 12 muffin cups and topping the cupcakes with chocolate buttercream.
Double it. For a larger version of this cake made to feed a crowd, double the recipe! Dust the top with powdered sugar before serving, or top with cream cheese frosting a la my gluten free pumpkin sheet cake recipe.
Flour substitute. If you don't need to keep gluten free, substitute 1 ½ cups of all-purpose flour for the gluten free flour.