Above are two cakes that I’ve made. The cake on the left is covered in fondant. The cake on the right was frosted with buttercream and has fondant decorations. You can make your frosting as smooth as mine with Viva paper towels. The only reason this method is called by name is because Viva paper towels are the only ones that have no pattern.
In order to make your frosting this smooth you will need to adequately prepare your frosting. You want your frosting thick enough to allow your spatula to stand without falling to the side of the bowl. Continue to add powdered sugar to your frosting until it is thick enough for your spatula to stand but not too thick. You want it just to the point that the spatula stands on its own. If you make it too thick, add vanilla coffee creamer to thin it slightly.
In addition, frostings that have Crisco or any generic shortening in them (even as little as 1/4 cup) will develop a “crust” on the cake that allows you to use this method without having the frosting stick to the paper towels.
Once your frosting is prepared, as described above, frost the cake and smooth it with the spatula as much as possible.
Let the cake sit for 15-20 minutes undisturbed. You’ll know it’s ready when you lightly touch the side of the cake and feel the “crust” on it. If it doesn’t develop a crust within 30 minutes then you may have a difficult time using this method.
After a crust has developed on the cake, place the paper towel on the cake and lightly smooth it out with your hand. If the paper towel seems to stick to the cake then dust it with powdered sugar before you begin and you should have no problems. The buttercream will soak up the powdered sugar so it won’t leave any residue on the cake.
Hope this helps you making the smooth frosting that you have been looking for.