Place the chocolate in a large bowl and set aside. Beat the egg yolks and 1/2 cup of sugar with an electric mixer on high speed until pale and creamy.
Bring the milk to a boil in a pan over medium heat. When the milk is simmering, remove from heat. Pour half of the hot milk into the egg mix, whisking continuously. Pour back into the pan with remaining milk and return to the heat. Stir constantly until it begins to thicken or coat the back of a metal spoon. Remove from heat and pour over the chocolate. Stir until the chocolate has melted and the mixture is thick and smooth.