Heat the milk in a medium sized nonreactive sauce pan, to just below boiling. Put the egg yolks in a small mixing bowl and whisk in the ½ cup sugar to blend. Slowly whisk the hot milk into the egg and sugar mixture to temper the egg yolks. Pour the hot milk-and-egg mixture back into the sauce pan and cook over moderate heat, stirring constantly with a wooden spoon, until the air bubbles that cover the surface have dissipated and the custard thickens enough to coat the back of the spoon thickly. Do not ever allow the custard to simmer or boil. Remove from heat, and stir in heavy cream. Strain through a fine-mesh sieve, and allow to cool completely.