Mash bananas in 1 bowl, not too much so they're liquid I like leaving it a bit chunky.
In another bowl mix cake mix, oil and eggs.
Add bananas and gently mix just until incorporated, do not over-mix.
If you want to add 1/4 c. chopped nuts and/or mini chocolate chips fold them in now.
Spoon into muffin liners, 3/4 of the way full and bake for 18 minutes or until they spring back when touched in the center. Do NOT overbake.
Remove and use a spoon to scoop muffins out of pan and on to a cooling rack, leaving them in the pan will overbake them and dry them out more.
Video
Notes
4 ingredient mini banana muffins
Follow instructions but bake in mini muffin pan for about 11 minutes or until top springs back, remove immediately from pan and on to wire rack to cool to remain moist.