Mash the bananas. With a potato masher or a fork, in a bowl. It doesn't need to be smooth: a few small pieces are even better.
Add the eggs. Both of them, and beat with the fork until well combined with the banana.
Stir in the butter. The 40 g, melted and warm, not hot, little by little while you stir.
Add the milk. The 100 ml, and mix.
Add the flour and mix just enough. The 120 g, with a spatula, only until there's no dry flour left. If you overmix, the waffles turn rubbery.
Let it rest for 5 minutes. It's normal for the batter to have a few small lumps: they disappear in the waffle iron. Meanwhile, heat up the waffle iron.
Pour the batter into the waffle iron. A fifth of it, in the center, and spread it out to near the edge. If the iron is clean, there's no need to grease it.
Close it and let it turn golden. Every waffle iron heats differently: go by the color, it should be nicely golden.
Take it out and repeat. With a wooden spatula, so you don't scratch the iron. Repeat with the rest of the batter until you have 5 waffles.