Preheat your oven to 425 degrees Fahrenheit. Mash your bananas then set aside for later. In a large bowl, cream together the butters and sugars with an electric mixer until light and fluffy. Then add the mashed banana and mix until combined.
Next, add your sour cream, milk, and vanilla extract. Mix until smooth then add your dry ingredients(flour, salt, baking soda, baking powder, cinnamon). Do not over mix this! Only mix until no more flour is visible. Your batter should be thick and have small lumps throughout.
Fold your add ins, whether that be the pecans or chocolate chips! Or if you want to do half a different flavor, just add the topping to the top of the muffins after you scoop them in the liners.
For the cinnamon streusel, combine melted butter, cinnamon, brown sugar, and flour in a small bowl until you get a crumbly mixture. Line your 12-count muffin pan with cupcake liners and fill the batter to the top to get tall domes! Then add streusel to the top of the muffin (or your other add ins). Bake for 5 minutes in your preheated oven (425) then, without opening the oven, lower the temperature to 350 and let the muffins cook for another 10-13 minutes until toothpick comes out mostly dry. Check them every 2 minutes after the 10 minute mark is up to make sure the do not get overcooked!
Let muffins cool on a wire rack then enjoy!