Prepare Your Cake

• To frost a cake with steam buttercream, the cake has to be carefully cooled and prepared. Thorough cooling is key: any warmth remaining in the cake will melt the buttercream unevenly, causing it to melt and slide around unevenly. Sharp edges can interrupt the smooth flow of melted frosting over the edges of the cake, so you'll need to round off the edges with a cake knife.

Prepare Your Work Area

• Frosting with steam buttercream is an inherently messy baking project, as it involves pouring a large amount of melted buttercream over the cake and allowing it to flow freely down the sides. To catch the overrunning excess and help control the mess, cut a cardbord "foot" for your cake and place it in a lipped tray on top of a wire rack, then place your cake on top of the foot.

Use the Right Tools

• Any buttercream recipe works for this method except for egg-based and whipped-cream versions, which change their hardening characteristics when heated. Since much of the buttercream will run off the cake as waste, you'll need a lot of it. To compensate, make about three times the amount of buttercream that a normally frosted cake would require. Make it easier to pour the large amount of heavy frosting by preparing it in a spouted plastic bowl with a sturdy handle.

Heat Carefully

• Heating buttercream can easily cause the frosting to separate. The treat's name derives from the easiest way to slowly, gently heat it to pouring consistency: steaming it a few inches over the surface of a pot of boiling water while stirring vigorously with the spoon. If the buttercream appears separated or oily, all is not lost: fix it by removing the bowl from the heat, thoroughly remixing the frosting, allowing it to cool and introducing it again to very gentle heat.

Pour the Frosting

• Slow, deliberate pouring is the key to a smooth coat of steam buttercream. The sequence, too, is key: start along the top of the curved edges until all the cake's sides are coated evenly, then pour a large amount of frosting onto the middle of the top of the cake. The best way to distribute the frosting is to jiggle the wire rack underneath, which encourages the frosting to flow down the cake's sides into your tray catchment. If the frosting hardens too much to pour smoothly, simply scrape the uneven areas, rewarm the frosting and try again. Cool thoroughly to harden the coat, then handle the finished product carefully; unlike similarly applied fondant, steam buttercream will harden to the gentle touch but will dent with moderate pressure.