1Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners.
2In a medium bowl, whisk flour, baking powder, baking soda, and salt together. Set aside.
3In a large microwave-safe bowl, melt butter in the microwave. Whisk in 1 cup of sugar. The mixture will be gritty. Let it rest for a couple minutes to slightly cool down. Stir in egg, yogurt, milk, and vanilla extract until combined. Slowly mix in dry ingredients until no lumps remain. Batter will be thick.
4In a small bowl, mix the remaining 1/2 cup of sugar and 2 teaspoons of 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. Top each cupcake with 1 more teaspoon of the cinnamon-sugar.
5Bake the cupcakes for 19-21 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely before frosting.
6For the frosting: 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, 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 is too thin or more cream if the frosting is too thick. Taste the frosting and add salt if frosting is too sweet (I like to add 1/4 teaspoon). Remove half of the frosting from the mixing bowl and transfer to a smaller mixing bowl. Beat 1 Tablespoon of cinnamon into half of the frosting.
7To swirl the frosting, 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 swirl tip. If you do not have a piping bag and tip, simply frost the cupcakes with both the vanilla and cinnamon frostings. There may be leftover frosting depending how much you use on each cupcake.
8Store cupcakes in an airtight container at room temperature for up to 3 days and in the refrigerator up to 5.