Preheat the oven to 350°F (175°C). Line all 12 cups of a standard muffin pan with paper liners. Lightly grease the liners if they are not labeled nonstick, as almond flour muffins can cling to plain paper.

Place the peeled bananas in a large mixing bowl and mash them with a fork until mostly smooth. A few small banana pieces are fine, but large chunks can create wet pockets in the baked muffins.

Add the eggs, maple syrup, avocado oil, and vanilla extract to the mashed bananas. Whisk until the eggs are fully incorporated and the mixture looks smooth and evenly blended.

In a medium bowl, whisk together the almond flour, baking powder, baking soda, cinnamon, and salt. Break up any compacted almond flour so the leavening is distributed evenly.

Add the dry ingredients to the banana mixture: Fold with a flexible spatula until no dry patches remain. The batter should be thick, moist, and scoopable rather than pourable. If using chopped walnuts or pecans, fold them in now.

Divide the batter evenly among the 12 prepared muffin cups, filling each approximately three-quarters full. Smooth the tops lightly with the back of the scoop so tall peaks do not brown too quickly.

Bake on the center rack for 20 to 24 minutes. The muffins are done when the tops are golden, the centers spring back lightly when touched, and a toothpick inserted in the center comes out clean or with a few moist crumbs. Wet batter on the toothpick means they need additional baking time.

If the tops are browning quickly but the centers are still wet, loosely place a sheet of aluminum foil over the pan and continue baking for 2 to 3 minutes. Do not seal the foil around the pan.

Place the muffin pan on a wire rack and let the muffins cool in the pan for 10 minutes. They will be delicate while hot, so removing them immediately can cause the sides or bottoms to break.

Transfer the muffins from the pan to the wire rack and cool for at least 5 more minutes before serving. The crumb will become firmer and easier to peel from the liners as the muffins cool.