Grease muffin tins or place cupcake liners in the tins.
Add the flour, eggs, brown sugar, baking powder, baking soda, vanilla, and salt to a blender or a bowl. Pulse until smooth.
If you are using just one mix-in (cinnamon for example) then you can stir it right into the blender/bowl (with a spoon- don't pulse) and then fill in cupcake liners ¾ full.
If using more than one mix in (such as chocolate chips, peanut utter, nutella) then fill the cupcake liners first, then sprinkle your mix-ins on top of each and stir in with a knife.
Bake for 10-15 minutes or until golden brown and an inserted toothpick comes out mostly clean. Allow to cool completely. Serve immediately.
These keep well for a few days. I just stored them back into the oven covered with aluminium foil.
Recipe by Box of Spice at https://boxofspice.com/2016/02/20-minute-whole-wheat-banana-muffins/