Preheat your oven to 220°C (fan bake) and line a 21cm square tin with baking paper.

Prick the puff pastry sheets with a fork, bake for 8 minutes, press down, and bake for another 8 minutes.

Mix custard powder, sugar, vanilla essence, and half of the milk until smooth. In another bowl, whisk the egg with the remaining milk.

Heat the remaining milk until boiling, then slowly add the custard mix while stirring constantly until thickened.

Stir in the egg mixture and softened butter into the custard, mixing gently until smooth.

Layer one baked pastry sheet in the tin, pour custard over it, and top with the second pastry sheet, pressing down gently.

Combine icing sugar, softened butter, and hot water to make icing; set aside some for cocoa mix.

Spread the white icing over the custard slice, drizzle cocoa icing for decoration, and use a skewer to swirl.

Cover and refrigerate for at least 5 hours or overnight before serving. Cut with a serrated knife.