Preheat the oven to 350°F (175°C) and prepare a 12-cup muffin tin with liners or lightly spray with nonstick cooking spray.
Add the eggs, milk, vanilla extract, creamy peanut butter, and bananas to the blender.
Add the optional monk fruit sweetener, baking powder, baking soda, salt, cinnamon (if using), and old fashioned oats on top of the wet ingredients in the blender.
Pulse or blend until the oats are ground and the batter is smooth and well combined.
Transfer the batter to a bowl and fold in most of the sugar-free chocolate chips, reserving a few to sprinkle on top.
Divide the batter evenly among the 12 muffin cups and top each with the reserved chocolate chips.
Bake for 15–20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and transfer them to a cooling rack; let cool slightly before serving.