Preheat the oven to 375°F (190°C). Grease a standard muffin pan with non-stick cooking spray.
In a large bowl, cream the butter and sugar until smooth. Add the egg, vanilla extract, milk, and sour cream, and mix until well combined.
In a separate bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt. Gradually fold the dry ingredients into the wet mixture until just combined.
Spoon the batter into the muffin tin, filling each cup about ⅔ full.
Bake for 25 minutes or until a toothpick inserted in the center comes out clean. Allow the muffins to cool in the tin for 1-2 minutes before transferring to a wire rack.
Melt the butter for the topping, then mix the sugar and cinnamon together. Brush each muffin with melted butter, then sprinkle generously with the cinnamon-sugar mixture.
Serve warm and enjoy!
