Grease tin and line with baking paper, leaving excess on the sides to lift out slice once set.
Place SAOs on bottom of tray to form a complete layer (cut SAOs to fit if necessary however 9 SAOS should fit well in a 23 x 23cm tin).
Place all of the milk except for ½ cup in a saucepan and heat until almost boiling.
Combine custard powder, sugar and remaining milk and add to milk in saucepan just before that milk boil, and then whisk over heat for a few minutes or until custard thickens.
Take off the heat and stir in the butter and vanilla extract.
Allow to cool for a few minutes before pouring the custard onto the SAO biscuits and then top with another layer of SAO biscuits.
Place into fridge to set and, once cool, combine icing sugar, milk and vanilla extract for the vanilla icing and top the slice with the icing – spreading the icing with the back of a spoon to encourage an even layer of icing.
Place back into fridge and allow to set for at least a few hours, ideally overnight. Dust with icing sugar if desired before serving.