Put the sugar in a pot and let it melt slowly on a low heat while stirring constantly until it is completely dissolved (it may only become slightly brown/too brown = bitter))
Add 3/4 of the milk to the melted sugar. Let the milk boil slowly so that the sugar dissolves again. The shock of the cold milk makes it freeze bizarrely, that's ok.
Mix the remaining milk with the vanilla pudding. Please do not add any more sugar here. When the milk-sugar mixture boils, simply pour in the mixed custard powder. Let the pudding boil for about 1 minute (but stir constantly).
Now the pudding must cool down (stir regularly, otherwise a "skin" will form).
Stir 1/2 cup of whipped cream into the cooled pudding, then the pudding is ready. For the marble effect do not stir too much, but rather fold in the cream.