Whisk flour, baking powder, baking soda, and salt in a large bowl. Set aside.

In a medium bowl, whisk melted butter with 1 cup of sugar. Add egg, sour cream, milk, and vanilla extract, and whisk until smooth.

Pour wet ingredients into dry ingredients and whisk until the batter is smooth with a few small lumps.

In a small bowl, mix ⅓ cup sugar with 2 teaspoons cinnamon. Set aside.

Spoon 2 tablespoons of batter into each liner. Sprinkle 1 teaspoon of cinnamon-sugar mixture on top. Add another tablespoon of batter, filling the liners about ¾ full, then sprinkle with ¼ teaspoon cinnamon-sugar. Swirl gently with a toothpick.

Bake for 20–22 minutes or until a toothpick inserted in the center comes out clean. Let cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Beat softened butter until smooth, then add confectioners’ sugar, heavy cream, and vanilla extract. Beat on high for 3 minutes. Add salt to taste. Transfer half of the frosting to a piping bag. Beat cinnamon into the remaining frosting.

Pipe cinnamon frosting onto the cupcakes, using a combination of vanilla and cinnamon frosting in the piping bag. Garnish with optional cinnamon sticks or mini snickerdoodle cookies.