Combine butter and chocolate in custard cup.
MICRO WAVE 45 SECONDS to 1 MINUTE on HIGH, or until melted.
Set aside.
Mix sugar, corn starch and salt together in l 1/2-quart casserole.
Gradually stir in milk.
Blend in melted chocolate mixture.
MICROWAVE 6 to 7 MINUTES on '8', or until thickened, stirring once with wire whip.
Stir a little of the hot mixture into egg yolk.
Blend warmed yolk into hot mixture.
MICROWAVE 1 MINUTE, 30 SECONDS on '8', or until custard coats a metal spoon.
Beat egg white in small bowl until stiff peaks form.
Gently fold into custard.
Fold in vanilla.
Cool.
For ovens without solid state heat control, MICROWAVE 5 to 6 MINUTES on HIGH, stirring three times.
After adding egg yolk MICROWAVE 1 MINUTE on HIGH, stirring once.