Although this icing can add a nice touch to any chocolate or vanilla cake, it is especially suited for topping the recipe #307692 posted here on Zaar. This recipe comes from The Complete Baking Cookbook (2007).
In a saucepan over medium heat, cook 1 cup of the sugar and water, stirring until sugar dissolves, then boil gently, without stirring, util temperature reaches 242 degrees F on a thermometer. Be sure to check to see that sugar granules are not clinging to sides of pan. If they are, gently brush off with a wet pastry brush.
Meanwhile, in a mixing bowl & with mixer on high speed, beat eggs just until foamy.
Remove sugar syrup from heat, reduce mixer speed to low & gradually pour syrup in a steady stream into the beaten eggs. DO NOT POUR SYRUP ONTO BEATERS, as this will spray sugar onto sides of bowl & it will harden.
After all sugar mixture has been added, increase speed to high & beat until mixture cools to room temperature, about 20 minutes. You can test this by touching bottom half of bowl.
With mixer running, add butter, a few pieces at a time, whipping until butter is incorporated & buttercream is fluffy & creamy, about 4 minutes.
Fold in orange zest by hand, then set aside.
Meanwhile, preheat oven to 350 degrees F.
In a bowl, combine almonds, egg white & remaining 1/2 cup sugar, then toss to coat evenly.
Spread onto prepared baking sheet in a single layer.
Bake in preheated oven until golden, about 8-10 minutes, stirring almonds once or twice to bake evenly.
Let cool completely, then fold into orange buttercream.