1/2 cup egg whites (about 8 egg whites)
1 cup sugar
3/4 pound unsalted butter, room temperature
1 teaspoon vanilla
Zest of two lemons
Zest of two limes
Juice of half lime
Pink and green food coloring for rosettes and leaves
To make the buttercream:
1. In the bowl of your stand mixer combine the egg whites and sugar. It will be very thick and grainy.
2. Put the bowl over a double boiler and stir it with a rubber spatula until the sugar is completely melted. This can take several minutes. You want to brush the sides down with the spatula to make sure all the sugar is melted and no grains are clinging to the sides.
3. Feel the egg mixture between your fingers to check for graininess. Once it is completely smooth and has no grains left, put it on your stand mixer and beat with the whip attachment on medium high speed.
4. Beat it until it is light, fluffy, glossy and the bowl feels just about room temperature. If the whites are not cooled off sufficiently it will melt the butter when you add it.
5. Once the egg whites are whipped and cooled, add the butter to it, 2 tablespoons at a time on medium speed.
6. Once you have finished adding the butter and it has mixed on medium speed for about a minute the buttercream should be creamy and glossy looking again. (set aside 1 cup of buttercream to use for piping the rosettes and leaves)
7. Add the salt, vanilla, zests and juice to remaining buttercream.