Place yolk, milk, sugar, cornstarch and salt in blender. Blend 30 secs to 1 minute.
Microwave on high up to 1 minute stirring every 10 seconds till thickened.
Whisk in butter and vanilla.
( could be eaten now - or - )
Place in 1 to 2 custard cups, place plastic wrap on surface and refrigerate till cool.

Optional - tuck banana slices into cream for a banana cream dessert or toss raspberry, strawberry, blueberry or peach over top for trifle type.