Preheat your oven to 180°C

On a biscuit tray, place pastry between 2 sheets of greaseproof paper and place another biscuit tray on top to keep the pastry from rising. Bake the pastry sheets in a moderate oven for 15-20 minutes or until golden brown

In a large saucepan, warm the milk, butter and sugar over a medium heat until the butter is melted. Blend the cornflour and custard powder in extra milk until smooth and pour into the saucepan

Turn the heat to high and bring the custard to a boil, stirring continuously to ensure no lumps form

When the custard is thick and smooth, remove the saucepan from the heat and stir in the egg and vanilla

Line the base of a 23cm square cake tin and place one sheet of the pastry on the base. While the custard is still hot, pour over the base

Place the other sheet of pastry on top of the custard, pressing down firmly. Set aside to cool slightly. Refrigerate until the custard is firm

Combine sifted icing sugar with softened butter, vanilla essence and 1-2 tbsp hot water in a bowl. Mix well until it forms a spreadable consistency

Spread over slice and cut into squares