To begin, preheat your oven to 350 and line a muffin tin with cupcake liners, or spray with non-stick spray.
To a large mixing bowl, add your butter and sugar and whisk them together. Next, add your vanilla, banana, and eggs and whisk them in too. Finally, while whisking, slowly add your milk into the wet ingredients.
To the same bowl, add your oats, cocoa powder, baking powder and salt. Whisk together till combined. Add your chocolate chips and stir them in.
Fill each muffin tin to the top with the batter. Bake in the oven for 18-22 minutes, or till the oatmeal cups have reached your desired consistency.
Let them cool for at least 15 minutes prior to removing, for easier removal (if not using cupcake liners). Then devour!
Stored in an airtight container in the fridge these are best within 4-5 days.