Preheat oven to 350°F. Line a 12-count cupcake tin with paper liners. Set aside.
In a medium bowl, whisk the flour with cream of tartar, baking soda, and salt; then set aside.
1⅔ cup all-purpose flour, 1 teaspoon cream of tartar, ¼ teaspoon baking soda, ¼ teaspoon kosher salt
Place the butter and 1¼ cups sugar in a large mixing bowl and beat on high speed with an electric hand mixer until light and fluffy, about 3-5 minutes.
½ cup unsalted butter, 1¾ cups granulated sugar
Scrape the sides, then add the eggs and vanilla extract and beat on high speed for 1 minute.
2 large eggs, 2 teaspoons pure vanilla extract
Scrape the sides again, then alternate by adding ⅓ of the flour mixture, then half of the buttermilk, mixing on low speed after each addition. Continue with the half of the remaining flour, followed by the buttermilk, then finish with the rest of the flour. Mix until just incorporated.
¾ cup buttermilk
Mix the remaining ½ cup granulated sugar and ground cinnamon together in a small bowl.
2 tablespoons ground cinnamon
Place about 2 tablespoons of the cupcake batter into each cupcake liner. Top the batter with 1 teaspoon of the cinnamon-sugar mixture.
Add one heaping tablespoon of the batter into each cup. Divide the remaining cinnamon sugar evenly over each cupcake.
Bake for 18-22 minutes, or until a toothpick inserted in the center comes out clean or with moist crumbs
Remove from the oven and cool completely before frosting.
1 recipe brown sugar frosting