Use this recipe to make our Birch de Noel. The simplest way to test whether the sugar has dissolved is by rubbing the mixture between your fingers; it should feel completely smooth with no remaining granules.
In a small heavy saucepan, combine 3/4 cup sugar, the water, and corn syrup; cook, stirring occasionally, over medium heat, until sugar dissolves. Raise heat, and bring to a boil, washing down sides of pan with a pastry brush dipped in water to prevent crystals from forming. Continue cooking without stirring until a candy thermometer registers 230 degrees, about 5 minutes.
Meanwhile, beat egg whites in the bowl of an electric mixer fitted with the whisk attachment on medium speed until soft peaks form, about 2 1/2 minutes. Gradually add remaining tablespoon sugar until combined.
Once sugar syrup has reached 230 degrees, immediately remove from heat; with mixer on medium-low, carefully pour mixture into egg-white mixture in a steady stream down the sides of the bowl to prevent splattering. Increase speed to medium-high, and beat until mixture is completely cool and stiff peaks form, about 7 minutes. Add vanilla, and beat until fully incorporated. Frosting should be shiny and smooth. Use immediately.