In a medium bowl, beat egg yolks and sugar until well blended. Pour into top of double boiler. Stir in half and half. Cook and stir over gently boiling water until mixture thickens and coats the back of a wooden spoon.
Set aside. Stir in butter. Stirring occasionally, cool on a rack to room temperature. Stir in heavy cream and vanilla. Chill overnight.
Pour into ice cream canister. Freeze in ice cream maker according to manufacturer's directions. Makes about 1 quart.