The easiest way to achieve a smoothly frosted cake is to begin with creamy smooth icing that is a thin consistency. Your icing should essentially have the
consistency of whipped cream; light and fluffy with no air bubbles. To achieve creamy smooth icing with a planetary action mixer (mixers that use just one
paddle), completely fill the bowl and cover the top of the paddle with icing. Make sure you are using the paddle attachment for buttercream and not the
whisk! Mix on low speed until there are no visible air bubbles; approximately 3 to 5 minutes. Do not over mix. If your mixer has standard beaters, mix with
the icing coming only half way up the beaters. Always beat your icing on the lowest speed to avoid air bubbles.
If a cake is frosted with creamy smooth icing, a smoothly frosted cake can usually be achieved by using only a spatula or icing tool held at a slight angle so just one side skims the surface of the icing. Smooth the sides first, then the top, working from the outside edge towards the center. Use long, smooth strokes that feather off just past the center. Clear the spatula after each stroke. This technique works beautifully with the right thin consistency icing, but practice makes perfect.
Other Methods
These methods are more forgiving of less than perfect icing consistency.
Paper Towel or Paper Smoothing Method
Certain brands of paper towels can be used to obtain a smooth surface on a buttercream icing that crusts. This means your icing surface will dry so that no
icing comes away on your hand when you touch it. This should be achieved by letting the iced cake sit at room temperature, as refrigerating it will cause
it to stay moist. How long it takes will depend on the humidity. The more humidity, the longer it can take, but let’s say 5 to 10 minutes.
Look for paper towels that don’t have a texture. There are only one or two texture-less paper towel brands on the market and the brand varies by locale. Be sure to keep your paper towels for smoothing icing in a container that will keep them from attracting any lint, i.e. a Large Ziploc bag.
To use, get the icing as smooth as you can with your spatula. Let the icing crust for approximately 15 minutes. Test a small area by rubbing your palm over a dry paper towel on the icing. If it does not stick, you may lay the paper towel over the icing and rub over it with your palm, the side of your hand or a fondant smoother or spatula. Don’t use your fingers as that will create ridges. This removes air bubbles and spatula marks creating a smooth surface. You may do the same technique with parchment paper or waxed paper but may have to wait longer for icing to be drier. Also, for even smoother icing, you can smooth again with the parchment paper or waxed paper after you’ve used the paper towels.
Water Smoothing
Water smoothing can be done one of two ways.
1. Frost the cake as usual and pre-smooth with a dry spatula. Dip a large spatula in hot water. Hold the spatula at a slight angle so just one side skims the surface of the icing. Smooth the sides of the cake first. Smooth the top of the cake from the outside edge of the cake towards the center. Use long, smooth strokes that feather off just past the center. Clear and rewet the spatula after each stroke. Be careful not to over-wet the icing. Colored icing may streak.
2. Use a spray bottle to slightly wet the icing, and then skim the spatula across the surface as described above. Clear the spatula after each stroke. Be careful not to over-wet the icing. Colored icing may streak.
*Important if using a spray bottle to smooth your icing, it’s imperative to empty the bottle every day. Rinse with a good dish soap and water and then allow to air dry. You don’t want to risk spraying “contaminated” water on your cakes. This is something the Health Department would be looking out for in a commercial kitchen.
The best tools and techniques to use are the ones that work best for you! I’ve worked with various cake stylists and each had her own favorite methods and favorite spatulas, etc. So your way can be the best way!!