1. Beat eggs and milk in a deep 1-quart non-metallic casserole until well combined. Stir in sugar and butter.
2. Heat, uncovered, in Microwave Oven 5 (6 1/2) minutes, stirring occasionally, until mixture thickens and boils.
3. Stir in vanilla and chocolate until chocolate has melted.
4. Cool to spreading consistency. Use to frost cake completely. Any remaining frosting may be refrigerated for use at a later time.