Preheat the oven to 350°F: Line the muffin pan with parchment paper liners. This recipe makes 18 muffins, so prepare two pans with muffin liners.
Mix the Wet Ingredients: In a stand mixer, combine mashed bananas and melted butter. Whisk to combine in a large mixing bowl. Then add sour cream, sugars, eggs, and vanilla. Whisk to combine.
Mix the Dry Ingredients: In a separate bowl, combine baking powder, baking soda, cinnamon, salt, binder/gum, and flour. Add the dry ingredients to the wet ingredients and mix until completely incorporated.
Fold in the Walnuts: Gently fold in the chopped walnuts.
Prepare the Crumble: Combine the sugar, butter, flour, and cinnamon in a small bowl and stir to combine.
Fill the Muffin Pans: Using a measuring cup or muffin scoop, add the muffin batter to muffin pans, filling each 3/4 full. Sprinkle the crumble atop the muffins before placing them in the oven.
Bake: Bake for 15 minutes. Check muffin doneness. Bake an additional 5 minutes or until golden brown. Let the muffins cool for 3-5 minutes before moving them to a wire rack to finish cooling.
Enjoy: Slather on the butter, grab your coffee, and ENJOY!