You do not have to be a professional baker to get a perfectly smooth buttercream surface on a cake. Smooth buttercream cakes can give a professionally decorated appearance, even when made at home. Use this technique for wedding cakes, birthday cakes and cakes you intend to cover with fondant. Start by using a buttercream recipe that is homemade, not canned. Buttercream should have no chunks and be smooth when spread on the side of a mixing bowl by a cake spatula.
Place a prepared cake on top of a decorator turntable. Place a large dollop of buttercream on top of the cake. Smooth the buttercream across the top of the cake and push it toward the edges using a cake spatula. Add more buttercream until the top of the cake does not show through the frosting.
Hold the cake spatula upright with the blade facing the base of the turntable as you apply icing to the sides of the cake. Apply enough buttercream to the sides of the cake so that you cannot see cake under the frosting. Do not worry about smoothing out the buttercream at this point. Use this method to simply get the buttercream on the cake surface.
Hold a pastry scraper against the side of the cake, slightly angled toward you. Turn the turntable in the direction of the bench top scraper. Do not move the scraper, but let it rest against the side of the cake so that it can smooth out the frosting. Continue to spin the cake around on the turntable until the frosting on the sides of the cake is even and smooth. Place excess buttercream that builds up on the scraper into a bowl.
Hold the cake spatula on top of the cake with the tip of the spatula in the centre. Angle the cake spatula up slightly, and spin the turntable to smooth the top frosting. Remove any excess frosting that builds up on the spatula blade. Gently flatten frosting peeking up on the edges of the cake by sweeping the cake spatula from the edge of the cake in toward the centre.
Allow the cake to rest and crust over for 10 to 30 minutes, depending on the temperature of the room. Test the surface by tapping it with your finger. If the buttercream sticks to your finger, it is not crusted over.
Place a piece of parchment paper over the crusted surface. Gently smooth the buttercream under the parchment paper with your hands. Start at the top of the cake and move to the sides. Remove the parchment paper. Decorate with further decorations or leave the cake as is.