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 sugar on medium speed for 1 minute. Add the egg, vanilla, mashed bananas and sour cream and continue beating for 30 seconds until fluffy.
Add the flour, baking soda, baking powder, and salt and beat 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 half of the batter between the prepared muffin tins (18 in total). Top with half of the crumb mixture followed by the remaining batter and lastly, top the muffin batter with the remaining crumb mixture.
Bake in the preheated oven for 20-25 minutes or until golden brown and a toothpick inserted into the center comes out clean. Enjoy!