Preheat the oven to 350℉. Grease the muffin tin(s) with non-stick spray or line with 18 muffin liners.
In a large bowl using an electric handheld mixer, cream the softened butter and brown sugar on medium speed for 1 minute. Add the egg, vanilla, mashed bananas, and sour cream and continue beating for 30 seconds until fluffy.
Whisk together the flour, baking soda, baking powder, and salt. Add to the wet ingredients and mix on low speed just until incorporated.
Stir together the brown sugar, flour, and cinnamon in a small bowl. Add the melted butter and stir to form a crumbly texture.
Distribute the batter between the prepared muffin tins (18 in total). Top with the crumb mixture distributed evenly between all the muffins.
Bake in the preheated oven for 20-25 minutes or until golden brown and a toothpick inserted into the center comes out clean. Enjoy!
Notes
You could use unsalted butter instead with ¼ teaspoon of additional salt added to the flour mixture. Add a pinch of salt to the crumble topping as well if using unsalted butter.
You could use plain unsweetened yogurt instead of sour cream.
Store fully cooled muffins in an airtight container with a layer of paper towel on the bottom to help absorb excess moisture.