Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners.

In a large bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside. In a medium bowl, whisk the melted butter and 1 cup (200g) sugar together. Mixture will be gritty. Whisk in the egg, sour cream, milk, and vanilla extract until combined.

Pour the wet ingredients into the dry ingredients and whisk until the batter is completely combined. Batter should be creamy and mostly smooth; a few small lumps are OK.

In a small bowl, mix the remaining 1/3 cup of sugar and 2 teaspoons cinnamon together. Set aside.

Spoon 2 scant Tablespoons of cupcake batter into each cupcake liner. Top with 1 teaspoon of the cinnamon-sugar mixture. Top with 1 heaping Tablespoon of cupcake batter, until the cupcake liners are about 3/4 full. Some of the cinnamon-sugar filling from the center layer may peek through the top layer of batter—that’s ok. Top each cupcake with 1/4 teaspoon of cinnamon-sugar. Swirl each gently with a toothpick.

Bake for 20–22 minutes, or until a toothpick inserted in the center comes out clean. Remove cupcakes from the oven and allow them to cool in the pan for 10 minutes, and then transfer to a wire rack to cool completely. Shake off any excess cinnamon-sugar that isn’t sticking to the tops, as that will prevent the frosting from sticking to the cupcakes.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat butter on high speed until smooth and creamy, about 1 minute. Add confectioners’ sugar, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then switch to high speed and beat for 3 full minutes. Add more confectioners’ sugar if frosting seems thin and greasy or more cream if the frosting seems too stiff. Taste, and add salt if frosting is too sweet (I like to add about 1/8 teaspoon). Remove half of the frosting from the mixing bowl and transfer to another bowl, or directly into your piping bag. Beat 1 Tablespoon of cinnamon into the remaining frosting in the mixing bowl.

Spoon half cinnamon frosting into the piping bag and half vanilla frosting into the piping bag. See above for a visual. Pipe onto cupcakes. I used a Wilton 1M piping tip. Garnish cupcakes with cinnamon sticks or mini snickerdoodle cookies, if desired.

Cover and store leftover cupcakes at room temperature for up to 1 day or in the refrigerator for up to 5 days. I recommend a cupcake carrier for storing and transporting decorated cupcakes.