*Omit the salt if using nut butter that is already salted.
Preheat the oven to 350F and generously grease a 9-inch by 13-inch baking dish with butter or coconut oil.
In a large bowl, use a fork to mash the bananas as smoothly as possible, though it's okay if there are still a few clumps. Add in the nut butter, eggs, baking soda, salt, vanilla, and cinnamon and stir well, creating a runny cake-like batter. Fold in the dark chocolate chips, if using.
Pour the batter into the greased baking dish and use a spatula to spread it evenly across the bottom of the pan. Bake at 350F until the top is golden and the center is firm, about 30 minutes. Allow the cake to cool in the pan completely before cutting and serving.
This cake is moist, so for best shelf life store it in the fridge for up to 2 weeks. (I think you could probably store it at room temperature for 2-3 days, but I prefer to keep things in the fridge to be on the safe side!)