Preheat your oven to 350°F (175°C).

In a medium bowl or the bowl of a stand mixer, combine the granulated sugar, cake flour, baking powder, baking soda, salt, and cinnamon.

Add the butter and beat on high speed using an electric mixer for about three minutes, or until the mixture becomes light and fluffy.

In a separate small bowl, whisk together the eggs, sour cream, and vanilla extract until the mixture is smooth.

Pour the egg mixture into the flour mixture and beat on medium speed just until everything is incorporated.

Gradually add the milk, mixing on low speed until fully combined. The batter will be quite thin—don’t worry, that’s exactly how it should be.

In a small bowl, stir together the ingredients for the cinnamon sugar filling.

Pour a thin layer of batter into each cupcake liner. Sprinkle a light layer of the cinnamon sugar filling over the batter.

Add more batter until the liners are about ¼ full, then sprinkle on another thin layer of cinnamon sugar.

Top with more batter until the cupcake liners are halfway full.

Bake for 14 minutes, then test for doneness with a toothpick—it should come out clean or with a few moist crumbs, but no wet batter. If needed, bake for another 2 minutes and test again.

Once the cupcakes are fully baked, remove them from the tin and let them cool completely on a wire rack or on the counter.

Using an electric mixer, beat the butter on high speed for about three minutes until light, fluffy, and airy.

Gradually add the powdered sugar, mixing a little at a time until well incorporated.

Add the cinnamon and salt, then continue beating for another minute. If you prefer a firmer buttercream, mix in more powdered sugar until you reach your desired consistency.

Pipe or spread the frosting over the fully cooled cupcakes. For a final touch, feel free to sprinkle a little extra cinnamon sugar on top.