Cookbooks often claim that 7-minute frosting is a cinch to make. Heat egg whites, sugar and corn syrup together and whisk them into a frenzy of sweet marshmallow delight. Unfortunately, the reality is that when it comes to making this sugary confection, every step has the potential for disaster. Cook it too long and you'll have a clumpy mess. Cook it too little and you'll have undissolved sugar, a syrup that won't whip and the risk of salmonella food poisoning. A candy thermometer and a few extra ingredients can solve most of your frosting-making woes.
Combine all the ingredients in your stand mixer bowl or an aluminum bowl. Add a pinch of cream of tartar if your recipe doesn't call for it. The cream of tartar helps stabilize the egg whites so they whip up more consistently and the frosting stays fresher longer.
Place the bowl over a saucepan of simmering water. The water level should be 1 to 2 inches beneath the bowl. This helps reduce the risk of overcooking.
Set a candy thermometer into the bowl. Stir the mixture constantly until the thermometer reads 165 degrees. Remove the bowl from the heat immediately. Using a candy thermometer ensures that the egg whites are cooked perfectly. This cooking temperature ensures that the frosting becomes thick and also destroys any harmful bacteria in the raw egg whites.
Use the whisk attachment of your stand mixer, rather than the paddle attachment. The whisk beats the eggs more quickly and adds more air so you get more volume. If you don't have a stand mixer, use the beaters of an electric hand mixer.
Whisk the frosting ingredients for 5 to 7 minutes, or until stiff peaks form and the frosting looks glossy. This frosting needs several minutes to set up properly. If you stop whisking it before it's really done, it may separate and become runny.
Spread the frosting over cakes or cupcakes with a spatula or cake knife. Cover with plastic wrap and store the cake in a cool, dry place. Don't store the frosted cake in the refrigerator because the frosting becomes weepy. Make and spread the frosting on the cake immediately before serving, if possible. This frosting tends to become hard and dense after a day or two.