Preheat your oven to 350°F (175°C) to ensure even baking.
If using muffin liners, place them in the muffin tin. Otherwise, lightly grease the tin.
In your blender, combine oats, cocoa powder, peanut butter, banana, egg, Greek yogurt (or sour cream), brown sugar, vanilla extract, baking powder, baking soda, and salt. Blend until smooth.
If using mix-ins, gently fold them into the batter using a spatula.
Pour the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake for approximately 15-18 minutes, or until a toothpick comes out clean.
Let the muffins cool in the tin for about 5 minutes before transferring to a wire rack.