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

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

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

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

Assemble the cupcakes: Spoon 2 scant tablespoons of cupcake batter into each cupcake liner. Then, sprinkle 1 teaspoon of the cinnamon-sugar mixture on top. Add 1 heaping tablespoon of cupcake batter, filling the liners about 3/4 full. It’s okay if some of the cinnamon-sugar from the center layer peeks through the top layer of batter. Top each cupcake with 1/4 teaspoon of cinnamon-sugar. Swirl gently with a toothpick.

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

Make the frosting: Using a handheld or stand mixer with a paddle or whisk attachment, beat the butter on high speed until smooth and creamy, about 1 minute. Add the confectioners’ sugar, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then switch to high speed and beat for 3 minutes. If the frosting seems too thin or greasy, add more confectioners’ sugar. If it’s too stiff, add more cream. Taste, and add salt if the frosting is too sweet (about 1/8 teaspoon is recommended).

Remove half of the frosting from the mixing bowl and transfer it to another bowl, or directly into your piping bag. Beat 1 tablespoon of cinnamon into the remaining frosting in the mixing bowl.

Assemble and decorate the cupcakes: Spoon half of the cinnamon frosting into a piping bag and the other half of the vanilla frosting into another piping bag. (See above for a visual guide.) Pipe the frosting onto the cupcakes. I used a Wilton 1M piping tip.

If desired, garnish the cupcakes with cinnamon sticks or mini snickerdoodle cookies.

Cover and store leftover cupcakes at room temperature for up to 1 day, or in the refrigerator for up to 5 days. A cupcake carrier is recommended for storing and transporting decorated cupcakes.