Heat the oven according to the temperature listed on the cake mix package. Prepare the cake pans by greasing and lightly dusting them with flour.
In a bowl, stir together the cake mix and cinnamon. Add water, oil, and eggs following the package directions. Mix until the batter is smooth.
Divide the batter evenly between the pans and tap them gently on the counter to release any air bubbles.
Place the pans in the oven and bake for 30 to 35 minutes. Check for doneness by inserting a toothpick into the center; it should come out clean.
Let the cakes sit in the pans for 10 minutes before moving them to a wire rack to cool completely.
For the frosting, beat the softened butter until creamy. Add powdered sugar a cup at a time, mixing thoroughly after each addition.
Stir in vanilla, a pinch of salt and heavy cream (or milk) gradually until the frosting reaches the consistency you want. Beat until light and fluffy.
After the cakes have cooled completely, spread frosting between the layers and stack them. Cover the top and sides with frosting and add any decorations you like.