Line 20 x 20 cm baking pan with baking paper. Layer biscuits along the bottom of the pan. Set aside.
Pour 300 ml milk into a bowl. Add in pudding then stir to combine. After that, fold in plain flour then stir again until smooth consistency. Set aside.
Pour the rest of the milk into sauce pan. Fold in sugar and vanilla sugar, then stir to combine. Bring to a boil on a medium heat, pour pudding mixture then reduce the heat to a low. Stir constantly until mixture thickens. Take off the heat and cover with aluminum foil immediately. Set aside for about 2-3 hours to cool completely.
In a big bowl cream soft butter. Fold in about 2 tbsp of cooled pudding and whip to combine. Repeat with the rest of the pudding mixture.
Spread the mixture over biscuits. After than, spread another layer of biscuits on the top of the pudding layer.
Refrigerate for at least 3 hours. Sprinkle with caster sugar before serving.