1 (18.25 ounce) box plain white cake mix
1 cup whole milk
1/2 cup (1 stick) butter, melted
3 large eggs
1 teaspoon vanilla extract
2 teaspoons ground cinnamon

Adjust the oven rack to the middle position and preheat the oven to 350 degrees F. Generously grease two 9-inch round cake pans with the vegetable shortening and coat with flour, shaking out any excess.

In a large bowl with an electric mixer on low speed, combine the cake mix, milk, melted butter, eggs, vanilla extract and cinnamon and mix for 1 minute. Stop the mixer and scrape the sides of the bowl with a rubber spatula. Increase the speed to medium and beat until well combined, 2 to 3 minutes. Divide the batter evenly between the prepared pans, smoothing the top with the rubber spatula.

Bake the cakes in the preheated oven until they are golden brown and spring back when pressed lightly, 27 to 29 minutes.

Transfer the pans to a wire rack to cool for 10 minutes. Run a knife around the edge of each pan. Invert the cakes onto a rack, the invert them again onto another rack so that the cakes are right-side up. Set aside to cool completely, about 30 minutes.

To frost, place 1 layer cake, right-side up, on a serving platter. Spread the top with about 1/3 of the frosting and smooth the top. Place the second layer, right-side up, on top of the frosted layer.

Spread the sides and top of the cake with the remaining frosting with clean, smooth strokes. Place the cake, uncovered, in the refrigerator until the frosting is set, about 20 minutes. (May cover with wax paper and refrigerate for up to 1 week.)

Cinnamon Buttercream Frosting
8 tablespoons (1 stick) butter, room temperature
3 3/4 cups confectioner's sugar, sifted
3 to 4 tablespoons whole milk
1 teaspoon vanilla extract
1 teaspoon ground cinnamon

In a large bowl with an electric mixer on low speed, beat the butter until fluffy, about 30 seconds. Stop the mixer and add the confectioner's sugar, 3 tablespoons of the milk, the vanilla extract and cinnamon. Blend on low speed until the sugar is incorporated, about 1 minute. Increase the speed to medium and beat until light and fluffy, about 1 minute. If the frosting seems too stiff, blend in up to 1 tablespoon additional milk.