Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together flour, sugar, baking powder, salt, and cinnamon.
In a separate bowl, whisk together melted butter, egg, milk, sour cream, and vanilla until smooth.
Add the wet ingredients to the dry ingredients and stir until just combined. Don't overmix.
Divide batter evenly among the prepared muffin cups, filling each about ⅔ full.
In a small bowl, mix together the 2 tablespoons sugar and 1 teaspoon cinnamon for the topping. Sprinkle evenly over the unbaked cupcakes.
Bake for 16-18 minutes, or until a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.