"Bastion City" style, via Roberta and Dave
Place a bowl over a pot of boiling water. Melt butter, stir in sugar.
Add remaining ingredients, stir until thickened.
Press into a 9x13” pan, chill 15 minutes.
Beat ingredients together, spread over base and chill.
Melt chocolate and shortening in microwave or double boiler. Spread over custard.
Chill slightly, until chocolate icing is no longer “wet-looking”, then slice. If chocolate gets too cold, it cracks when sliced.
Refrigerate or freeze.