Adjust an oven rack to the lower middle position. Preheat the oven to 350°F. Grease two 8 by 4-inch loaf pans and line each with a parchment paper sling.
In a medium bowl, whisk the 3 cups oat flour, ½ cup almond flour, 1 ½ teaspoons cinnamon, 1 teaspoon ginger, ½ teaspoon nutmeg, 2 tablespoons baking powder, and 1 teaspoon fine sea salt. until combined.
In a large bowl, vigorously whisk the 4 large eggs until pale yellow and doubled in size, 2 to 3 minutes. Add the 1 ½ cups granulated sugar and 1 ½ cups brown sugar, and whisk well to combine. Whisk in the ½ cup olive oil, 8 tablespoons unsalted butter , 2 teaspoons pure vanilla extract, and 1 15-oz can pure pumpkin puree until smooth. Fold in the flour mixture until well combined. Add the 1 ½ cups chocolate chips now or wait to add to one loaf.
Divide the batter equally between the prepared pans, stir 1 cup chocolate chips into one of the loaves if using, then place in the oven and bake for about 1 hour, until a tester inserted in the center comes out clean.
Remove the loaves from the oven, place on a wire rack, and cool in the pans for 20 minutes. Using the parchment sling, gently lift the loaves out of the pan, peel off the paper, and let the bread finish cooling on the wire rack.
Notes
Choose the right pumpkin. Always use pure pumpkin puree! (Or easily make your own with a pumpkin using my homemade method.) Pumpkin pie filling or “sweetened pumpkin,” which I almost bought the other day by mistake — yikes — will add extra sugar.
Adjust the sweetness. I do find this recipe to be a tad on the sweeter side, which on some days I love and others it just feels a bit much. When I want to go a little less sweet, I reduce the amount of sugar to 1 cup of brown sugar and 1 cup of granulated sugar, and that suits me fine. I’ve also done 1½ cups brown sugar and ½ cup maple syrup. You can experiment or make the recipe as written. Just know you may wish to reduce the sweetener, too.
Use your favorite gluten-free flour. I prefer single-origin gluten-free flours here, but you can use a gluten-free baking blend or my homemade gluten-free flour mix instead.
Try different mix-ins. Stir ½-1 cup of semisweet chocolate chips into one loaf, or add walnuts for crunch. Sarah calls for pecans in the original recipe, so those work beautifully, too. A combination of nuts and chocolate is especially good.
Make your own pumpkin spice blend. When I’m feeling lazy, I use my homemade pumpkin spice mix instead of measuring out the spices individually. It doesn’t much matter as long as there’s a decent amount of cinnamon to give the bread a fall-ish kick.
Bake mini loaves for gifting. Since the recipe makes two loaves, you can divide the batter among mini loaf pans for a lovely homemade holiday gift.