Make the frosting shortly before assembling the cake.Plus: More Dessert Recipes and Tips
In a small saucepan, combine the 2/3 cup of sugar with the water. Stir over low heat until the sugar dissolves. Using a brush dipped in cold water, wash down any sugar crystals from the side of the pan. Boil the syrup without stirring over moderately high heat until the syrup reaches 240° on a candy thermometer, about 7 minutes.
Meanwhile, in the bowl of a standing electric mixer fitted with the whisk, beat the egg whites with the cream of tartar at medium-high speed until they hold soft peaks. Add 2 tablespoons of the sugar and beat until the whites are firm. With the machine on, drizzle the boiling syrup down the side of the mixing bowl, avoiding the whisk, and beat at low speed for 3 minutes until stiff, glossy peaks form when the whisk is lifted. Beat in the orange zest and vanilla and let cool for 10 minutes. Use the Meringue Frosting as soon as possible.