In a large bowl, add in oil, brown sugar, and sugar. Stir everything together.
1/2 cup Oil, 1/2 cup Brown Sugar, 1/2 cup Sugar
Add in eggs and vanilla. Mix together.
2 Eggs, 1 teaspoon Vanilla
On a plate, sprinkle the cinnamon over the bananas and mash them up to a chunky* paste.
4 Bananas, 1 teaspoon Cinnamon
Add banana mixture to the wet ingredients and stir together.
To the wet ingredients, add in flour, oats, baking soda, and baking powder. Stir everything together.
1 teaspoon Baking Powder, 1/2 teaspoon Baking Soda, 1 1/2 cups Flour, 1 cup Oats
To get the most rise in your muffins, cover and refridgerate for an hour.
Preheat oven to 400 degrees.
While the oven is preheating, make the streusel topping.
Melt butter in a microwave-safe bowl in 30 second increments until it's mostly melted. There will still be some small chunks of butter and that's okay!
4 tablespoons Butter
Then mix in flour, oats, brown sugar, chopped walnuts, cinnamon, and salt.
2/3 cup Flour, 2/3 cup Oats, 1/3 cup Brown Sugar, 1/4 cup Walnuts, 2 teaspoons Cinnamon, 1/4 teaspoon Salt
Set streusel aside. Fill up muffin tins using a large cookie scoop. Then top with some streusel.
Bake at 400 for 5 minutes, then reduce the heat to 350 and bake for another 15 minutes.
Let cool slightly, then serve and enjoy!