Pre-heat the oven to 350 degrees Fahrenheit. In a large bowl, sift the coffee flour and all-purpose flour together. Repeat 2 to 3 times to make sure the flours are evenly combined.
Add the sugar, baking powder, baking soda, cinnamon, nutmeg, and salt. Mix well.
In a separate bowl, combine the mashed bananas, egg, black coffee, vanilla extract, and olive oil. Stir until combined. Slowly add to the dry ingredients, mixing frequently. Fold in walnuts or chocolate chips if you’d like.
Scoop the batter into a greased doughnut pan, filling up ¾ of each cavity. Bake for 10 to 12 minutes or until toothpick comes out clean.
In a small saucepan over low-medium heat, sauté the garlic and onion in olive oil. Cook for 5 minutes.
Add the remaining ingredients. Mix well, making sure the coffee flour completely dissolves.
Simmer until the sauce thickens, about 25 to 30 minutes. Mix frequently and avoid letting it boil. Do a taste test just before removing the sauce from the heat. If necessary, add more sweetener or vinegar, depending on your preference. Serve as a dip or marinade.