Preheat oven to 180˚C bake. Line two oven trays with baking paper.
Line a 23cm square cake tin with baking paper (ensure to leave overhang at the edges to lift the slice out with later).
Place a pastry sheet on each oven tray. Prick each pastry sheet all over with a fork and bake for 25 minutes, until golden brown.
You may want to switch the position of the trays halfway through cooking. As soon as they come out the oven, use a spatula to gently press each pastry sheet down. Leave to cool.
Mix custard powder to a smooth paste with a little of the milk and set aside. Add the rest of the milk to a medium-sized saucepan and heat until milk just starts to bubble around the edges.
Whisk in the Chelsea Raw Caster Sugar, followed by the custard powder paste and continue to heat and whisk until boiling and thickened (3-5 minutes). Remove from heat and stir in vanilla essence.
Use a serrated knife to trim the pastry sheets to fit in the prepared cake tin, if necessary.
Place one pastry sheet in the base of the tin and top with the custard. Place the remaining pastry sheet on top (choose the flattest side to face up) and press down gently. Refrigerate for 1 hour.
Combine Chelsea Vanilla Flavoured Icing Sugar and butter with a little water, mixing until you have a smooth icing. Spread over the top pastry layer and refrigerate for a further 30 minutes, until cold and set. Use a serrated knife to carefully cut into squares.