Preheat your oven to 425°F and line a muffin pan with muffin liners.
Add the 4 medium bananas to a large mixing bowl and mash them well with a fork. You should have about 1 1/2 cups of mashed banana.
Add the melted vegan butter, applesauce, vanilla extract, and plant milk to the mashed bananas. Whisk everything together really well until fully combined.
In a separate bowl, add the flour, baking soda, baking powder, cinnamon, and nutmeg. Whisk everything together really well to make sure the baking soda and baking powder are evenly distributed throughout the flour.
Add the dry ingredients to the wet ingredients and gently mix just until you no longer see any dry flour. Be careful not to overmix the batter. A few small lumps are completely fine.
Add a heaping 1/4 cup of batter to each muffin liner, dividing the batter as evenly as possible.
Place the muffin pan in the oven and bake at 425°F for 4 minutes.
Without opening the oven door, reduce the temperature to 350°F and continue baking for another 16 minutes.
Check the muffins by inserting a toothpick into the center. They’re ready when it comes out clean or with just a few moist crumbs.
Remove the muffins from the oven and let them cool in the pan for a few minutes before transferring them to a cooling rack.