Foamy Sauce for Puddings

1 cup confectioner's sugar
1 tsp.vanilla
1/3 cup butter
1 egg white
1 egg yolk

Sift sugar; next beat butter until soft and smooth; add sugar until well blended. Beat in egg yolk and vanilla and place mixture in double boiler over boiling water. Heat until yolk slightly thickens, stirring constantly. Beat egg white till stiff; and fold into mixture. Serve hot.