Mix together cinnamon with ⅓ cup sugar. Butter muffin tin or use liners; set aside.
In an electric mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy. Add eggs, and beat to incorporate. In a medium bowl, whisk together flour, baking soda, and salt.
Add to the butter mixture, and mix until just combined. Add bananas, sour cream, and vanilla; mix to combine.
To the prepared pan add half the batter you would usually add to a muffin tin - about 2 heaping Tablespoons. Sprinkle some prepared cinnamon sugar to each well and gently stir in so cinnamon sugar is not sitting on bottom. Then add a little more cinnamon sugar to form a layer. Top with the remaining ½ scoop of batter. About 2 Tablespoons. Top with remaining sugar mixture.
Bake until a cake tester inserted into the center of a muffin comes out clean, about 18-20 minutes.