Portion the sugar and corn syrup in a small saucepan, drizzle a few tablespoons of water over the sugar, and stir together until the sugar is all moistened, it will have a wet sand feeling about it. Cook the sugar to the softball stage of 235 F using a candy thermometer, use a pastry brush to lightly water down the inner sides of the sauce pan to prevent crystallization. When the sugar is done, turn off the burner (if gas), or move the pan off the burner (if electric).
Begin to whip your egg whites with a whip attachment in a stand mixer. When the meringue begins to look opaque, but is not to any sort of 'peak' stage yet, slowly pour the cooked sugar syrup down the side of the mixing bowl, as the mixture is whipping on a medium speed. Allow the mixture to whip until glossy medium peaks form and the meringue is cool to the touch.
Turn the mixer on a low speed, and add the butter, one stick at a time. Mix until the mixture comes together, don't be scared! The mixture will separate and look terrible before it comes together again, which It will- I promise. Be patient.
Once the mixture is smooth and wonderful-looking, add the vanilla and whip just until combined.
Makes about 4 cups - more than enough to frost a two layer 9-inch cake