Heat in the top of a double boiler over hot water, whisking constantly until mixture thickens. Remove from heat, cover with plastic wrap and chill. Yields 1 cup. Whisk together egg yolks and sugar. Stir a small amount of the hot mixture into the yolks.
Return yolks to hot mixture and cook gently until mixture lightly coats the back of a spoon.
Remove from heat, cover with plastic wrap and chill. Yields 1 cup.