Put the egg and egg yolks in the bowl of a stand mixer and whip until pale yellow and frothy.
While eggs are whipping, cook the sugar and water in a saucepan over low heat until it reaches the softball stage (238 degrees).
While mixer is still running, pour hot sugar syrup into the egg mixture. Continue to whip until it's cooled to room temperature.
While mixture is whipping/cooling, melt chocolate in saucepan over low heat.
Once egg mixture has cooled, add butter a tablespoon or two at a time. It will thicken as more butter is added.
Add the vanilla and melted chocolate.