Preheat your oven to 325. Line 2 12-well muffin pans with 20 to 24 cupcake liners.

In the bowl of your stand mixer fitted with a whisk attachment, beat the butter for 2 minutes on high speed.

Slowly add the granulated sugar. Cream together for an additional 5 minutes until very pale yellow and fluffy.

Add the eggs and egg yolk 1 at a time, combining well after each addition and scraping down the sides and bottom of the bowl as needed.

Turn your mixer down to its lowest speed and slowly add the flour in 2 batches.

Add the salt, baking powder and baking soda. Be careful not to overbeat.

Add the sour cream, oil and vanilla extract and mix until combined. The finished batter should be pale yellow and have a silky texture.

In a small bowl, whisk together all ingredients until well combined. Set aside.

Add cupcake batter to each liner filling only ⅓ of the way.

Drizzle a teaspoon of cinnamon swirl over cake batter in each liner.

Add more cupcake batter to each liner filling ¾ of the way.

Using a toothpick or skewer, swivel the batter a little to work the swirl through the cupcake.

Bake cupcakes for 17-21 minutes or until a toothpick inserted into the center of the cupcake comes out clean.

Let the cupcakes cool in the pan for 10 minutes, then transfer to a wire rack. Let cool to room temperature. Lightly cover the cupcakes with foil or plastic wrap so they do not dry out.

Clean your stand mixer bowl and whisk attachment.

Add the butter, cream cheese and confectioners sugar and mix on low speed until just combined.

Turn your mixer speed to high and beat for 2 minutes.

Add heavy cream, vanilla extract and ground cinnamon and mix until the buttercream is fluffy.

Once cupcakes have cooled completely, frost them with butter cream.