Gently boil water in a double boiler
In the top of the double boiler, add hot chocolate mix, sugar and butter. Stir until fully combined
Add in white and milk chocolate chips (and dark if desired)
As the chocolate chips begin melting, add 1/2 of the evaporated milk and stir until smooth
Turn off heat and leave to cool to room temperature (approximately 8-10 minutes)
Add ice, chocolate mixture, and remaining half of the evaporated milk into a blender and blend completely
Pour into glasses and top with whipped cream, sprinkles, chocolate chips or marshmallows as desired