Remove the butter and the eggs from the fridge and let them sit out at room temperature for 30 minutes.
Position the oven rack in the middle of the oven and preheat the oven to 350F. Grease or butter the loaf pan.
Mix:
In the bowl of a stand mixer or in a large bowl using a hand mixer, beat the butter and sugar for 2 to 3 minutes until the butter is airy and fluffy.
Add the eggs, one at a time, and mix for about 1 minute each. Add the vanilla and mashed bananas to the bowl and mix until combined.
Add the pancake mix and the cinnamon (if using) to the bowl. With the mixer on low speed or with a spatula, incorporate the dry ingredients into the wet mixture until just combined. The batter might be a bit lumpy from the bananas.
Pour the batter into the prepared loaf pan, and using the back of a spoon or a spatula, spread the top to even it out.
Bake:
Bake it in the preheated oven for 50 to 55 minutes, until the middle is no longer wet and when a toothpick inserted in the middle comes out clean. Around minute 35, it will have risen as much as it's going to rise. This is a good time to cover the banana bread with foil until the end of baking time to avoid burning the top.
Remove the banana bread pan from the oven and let it cool down to room temperature before removing it from the pan and slicing it.
Notes
This can be made in a 9x9 square baking dish or a disposable tin tray. Reduce the baking time to 22-27 minutes.