Grease a 3 cm-deep, 20 cm (base) square cake pan and line base and sides with baking paper, extending 3 cm over all sides. Place 9 biscuits, trimming to fit if needed, in a single layer, in pan to cover base.
Place milk, thickened cream, unsalted butter, vanilla, sugar, cinnamon, nutmeg and cloves in a medium saucepan. Bring to a simmer over medium heat.
Meanwhile, whisk cornflour and yolks in a large bowl. Gradually add warm cream mixture to cornflour mixture, whisking continuously until smooth and combined. Return cream mixture to saucepan and heat over medium-low heat. Cook, whisking constantly, until mixture boils and thickens (do not overheat or mixture will separate).
Pour warm custard over biscuits. Top with another layer of 9 biscuits, with the glazed side up. Refrigerate for 4 hour 30 minutes or until chilled and set.
To make the icing, place icing sugar in a large bowl. Pour in melted butter, 1 tablespoon of warm water and the vanilla. Stir until smooth (see notes).
Remove slice from pan and cut slice into squares, using shape of biscuits as a guide.
Spread the icing over the slice pieces and scatter with gold stars.
Tips & Hints:
You may need a little more warm water to get a smooth icing consistency. If so, add water gradually (a tablespoon at a time) until smooth.
This slice is best left in the fridge for several hours or overnight to allow biscuits to soften and custard to set.
Edible gold stars are available in the baking aisle of most supermarkets.