Mash that banana like you mean it. Take your sad, overripe banana and mash it up in a large bowl. You want it smooth-ish, but don't stress about a few lumps—texture is character. If you end up with more than 1/4 cup, just toss the extra. We're not running a banana charity here.
Build your base. Add the melted butter (or oil), brown sugar, and mashed banana to your bowl. Mix it all together until it looks like it wants to be friends. The brown sugar might clump a bit—that's fine, just keep stirring.
Add the wet stuff. Toss in your egg yolk and vanilla extract. Mix it well until everything's combined and looking smooth. This is your flavor foundation, so don't half-ass it.
Dry ingredients, assemble! Add the flour, salt, baking soda, and cinnamon all at once. Stir until everything's combined and there are no streaks of flour hiding in there. The dough will look a little wet—that's totally normal and actually what we want.
Chocolate chip time. Fold in those chocolate chips like you're tucking them into bed. Don't overmix; just get them distributed throughout the dough. Now here's the important part: cover the bowl and let it sit for about 15 minutes while your oven preheats. This lets the flour hydrate and makes for better cookies. Trust the process.
Bake those beauties. Line a cookie sheet with parchment paper and preheat your oven to 350°F. Once it's ready, use a cookie scoop or spoon to drop about 1-inch dough balls onto the sheet, spacing them about 2 inches apart. The dough will still be a bit wet—don't panic, that's how it's supposed to be.
The perfect bake. Bake for 10-11 minutes, until the edges are set and the cookies have puffed up, but the centers still look slightly underbaked. This is crucial—nobody wants dry cookies. They'll continue cooking on the hot pan after you take them out.
Practice patience. Let them cool before eating. I know it's torture, but hot cookies fall apart and burn your mouth. Give them at least 10 minutes to get their act together.