Pour custard, milk and Baileys Original Irish Cream in the jug of a blender and process until smooth.
Prepare the hot fudge sauce by melting chocolate, cream and butter in a small heavy bottomed saucepan on low heat for a couple of minutes. Remove from heat when chocolate is melted. Stir until glossy and smooth.
To serve, pour Baileys shake in two tall 500ml capacity footed glasses. Top with whipped cream and marshmallows. Toast the marshmallows using a cooking blow torch. Drizzle with the hot fudge sauce and serve immediately.