Preheat the oven to 350°F. Generously spray a standard 12-cup muffin pan with nonstick cooking spray.
In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar with a hand mixer or stand mixer for 2–3 minutes, until light and creamy.
Add the eggs, milk, sour cream, and vanilla extract. Mix until combined.
Add the flour, cinnamon, baking powder, baking soda, and salt. Stir by hand just until combined, being careful not to overmix.
Divide the batter evenly among the muffin cups, filling each about ? full.
Bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the pan for 5 minutes, then carefully transfer them to a wire rack.
For the topping, melt the butter. In a separate small bowl, stir together the granulated sugar and cinnamon.
While the muffins are still slightly warm, brush each muffin with melted butter, then coat all sides with the cinnamon-sugar mixture.
Serve and enjoy.
Notes
Do not use cupcake liners, as the cinnamon-sugar topping is applied to all sides of the muffins. Generously grease the muffin pan to make the muffins easier to remove.Store leftover muffins in an airtight container at room temperature for up to 5 days. For best results, microwave leftover muffins for 15–20 seconds before serving.