Preheat the oven to 425°F (218°C).
Make the crumble topping: In a small bowl, mix the brown sugar, cinnamon, and butter until combined. Add the flour and use a fork to mix and fluff together until crumbs begin to form. Be careful not to overmix into a paste. Place the crumbs in the refrigerator until it is time to top the muffins. *This makes a generous amount of topping, so don't be afraid to use a lot!
Mix wet ingredients: In a large bowl, whisk together the mashed bananas, melted butter, oil, and sugar until smooth with no visible clumps. Add the eggs and vanilla until fully combined. Finally, add the sour cream until it is no longer clumpy.
Mix dry ingredients: In a separate bowl, whisk the all purpose flour, baking powder, baking soda, salt, and cinnamon.
Fold ingredients: Gently fold the dry ingredients into the wet mixture just until combined. Be careful not to overmix. The batter should be thick but scoopable.
Lightly chop the pecans so that the pieces are not too large. Sprinkle with a light dusting of flour and gently fold into the muffin batter. Let the batter rest for 15 minutes.
Fill the pan: Line the muffin tin with paper liners or parchment paper, or spray with baking spray. For regular muffins, fill each liner about halfway. For larger, bakery style muffins fill only half of the muffin spaces (alternating) all the way to the top.
Toppings: Remove the crumble topping from the refrigerator and generously sprinkle over the batter.
Bake: Bake at 425°F (218°C) for 5 minutes, then reduce the oven temperature to 350°F (180°C) without opening the oven. Bake for 14-16 minutes or until a toothpick poked through the center comes out clean.
Cool: Remove from the oven and cool in the pan for 5 minutes. Transfer to a wire baking rack to cool all the way.