Preheat the oven to 350°F (175°C). Line a mini muffin tin with mini cupcake liners.

In a small bowl, whisk together the flour, old fashioned oatmeal, baking soda, and salt until evenly combined. Set aside.

In a large mixing bowl or the bowl of a stand mixer, beat the softened butter until light and fluffy.

Add the mashed banana to the butter and beat until incorporated.

Add the packed light brown sugar, granulated sugar, and vanilla extract; beat until the mixture is creamy and smooth.

Add the eggs one at a time, beating well after each addition.

Gradually add the flour–oatmeal mixture to the wet ingredients, mixing on low speed or folding by hand just until no large streaks of flour remain. Do not overmix.

Stir in the chocolate chips until evenly distributed.

Using a teaspoon or a small scoop, drop about 1 teaspoon of batter into each mini cupcake liner. Fill each liner nearly to the top, leaving a little room for the muffins to rise.

Bake for 13 to 18 minutes, or until the tops are golden brown and set. A toothpick inserted into the center should come out with a few moist crumbs but no raw batter.

Let the muffins cool in the pan for about 5 minutes, then transfer them to a wire rack to cool further before serving.