In a large bowl, whisk the melted butter with the brown sugar, egg, and pumpkin puree until smooth. Stir in the oat flour, baking soda, spices, and salt.
Transfer the dough to an airtight container and refrigerate it for at least two hours, preferably overnight, and up to 24 hours and preferably overnight.
Line a baking sheet with parchment and preheat the oven to 350℉. Form the dough into equal-sized balls, about 2 tablespoons each. Place them on the baking sheet with 2 inches in between the cookies, then gently flatten the cookies slightly with the palm of your hand.
Bake for 9-12 minutes or until the edges are browned and the centers look puffed and slightly cracked. Remove the cookies from the oven and cool on the baking sheet for 2 minutes, then transfer to a wire rack to finish cooling.
Notes
Rest the dough - The longer you can let the dough rest in the fridge, the more flavorful the cookies will be. Store it in an airtight container so it doesn't pick up any fridge odors for up to 3 days. I like to let my dough chill for at least 24 hours before I bake the cookies.
Use gluten free flour - You can use your favorite gluten free all-purpose flour for the oat flour if you wish. Measure out 180 grams of flour for the best accuracy.
Other spices - Easily swap pumpkin pie spice for the 1 teaspoon of total spice. Or, try other spices like cardamom or a pinch of cloves.
Finish with cinnamon sugar - Blend 1 teaspoon of cinnamon with 2 teaspoons of granulated sugar. Roll each ball of dough in the cinnamon sugar before adding it to the baking sheet.