Mix sugar, cocoa powder and custard powder together ensuring there are no lumps. Then add enough measured milk to make a thinnish paste (1/4 cup maybe or less).
Grate chocolate if using and set aside.
In a saucepan on medium, heat remaining milk until chill is taken off then slowly while whisking pour in the custardy paste. Keep whisking until the custard just starts to gently boil. Be careful it doesn't stick to the bottom of the pan or burn.
Turn off heat and whisk in grated chocolate. Take off heat.
Either divide up into individual bowls and let set or leave in 1 big bowl to serve later. Refrigerate.
OR serve warm/hot.
Can serve hot or cold with fruit, or pudding or cake.