Mix-ins: Up to 1 cup of fresh or frozen fruitsuch as blueberries, raspberries, or diced strawberries; if using frozen, no need to thaw OR 1/2 cup of smaller, harder mix-ins, such as chopped dried fruit, nuts, and chocolate chips
Preheat the oven to 350 degrees F. Line 10 cups in a standard 12-cup muffin pan with paper liners.
Place the coconut oil in a medium, microwave-safe bowl. Heat just until melted, then set aside to cool to room temperature.
In a large, separate bowl, whisk together the almond flour, baking soda, salt, and cinnamon.
To the bowl with the coconut oil, add the eggs, banana, maple syrup, vanilla, and vinegar. Whisk until smooth. If the oil resolidifies, warm it in the microwave in short, 5-to-10-second bursts, until it melts again.
Make a well in the center of the dry ingredients, then pour in the wet ingredients. By hand with a rubber spatula or wooden spoon, stir just until combined and the flour disappears. Fold in any mix-ins.
Scoop the batter into the prepared muffin cups, dividing it evenly between the 10 muffins (they will be filled almost all the way to the top). Bake for 15 minutes, then loosely tent the top of the pan with foil to keep the tops from browning too quickly. Continue baking for 10 to 15 additional minutes (25 to 30 minutes total) until the edges are light golden, a toothpick inserted in the center of a muffin comes out clean, and the centers feel set when lightly touched. Place the pan on a cooling rack and let the muffins cool in the pan for a 10 full minutes, then gently transfer the muffins to a wire rack to finish cooling completely.
*Be sure to use blanched, finely ground almond flour without the skins. This is the brand that I use.
Don't skip the muffin liners. These are very moist muffins, and even with baking spray, they like to stick. (Silicone muffin pans might work without liners but I haven't tried to verify.)
If using frozen fruit: rinse briefly to remove any ice crystals, then add right to the batter—no need to thaw completely. Bake as directed, adding a few minutes to the baking time as needed.
Store muffins in an airtight container lined with paper towels in the refrigerator for up to 5 days. To freeze, see my guide for How to Store Muffins.