Preheat the oven to 375 degrees F and grease a standard muffin pan with nonstick spray. (I use classic Pam spray.)
Add the butter, bananas, eggs, and vanilla to a large bowl. Stir to combine.
Stir in the flour, sugar, baking soda, cinnamon and salt.
Divide the batter among the prepared baking pan, using about 1/4 cup batter In each cup. Bake for 14-16 minutes, or until golden brown and a cake tester inserted into the center comes out cleanly.
Transfer muffins from the pan to a wire rack to cool.
Serve warm or at room temperature.
Store any leftovers, once fully cooled, in an airtight container at room temperature for 3 days, in the fridge for 5 days. Or freeze in a zip top storage bag with as much air removed as possible for up to 6 months. Thaw in the fridge or at room temperature.
Serve chilled or slightly warmed.
Use all all-purpose flour if desired. They may need 16-18 minutes in the oven.
Use plain whole milk yogurt instead of the butter.
Add 1/2 cup chocolate chips to the batter if desired.
Omit the sugar if desired, but be sure to use VERY ripe bananas so the muffins taste sweet. (They will not be super sweet, but it's an option.)
Mash the bananas very smooth with a fork or a potato masher. (This is a good step for the kids to help with!)
Spread on some all fruit jam, a thin smear of nut butter, or cream cheese if desired.
Gluten-free: Use a 1:1 style of gluten-free baking mix.
Dairy-free: Use melted coconut oil in place of the butter.
Egg-free: Omit the eggs and add 1/2 cup milk to the batter.