Cooking oil spray, to grease
200g packet lattice biscuits
20g butter, chopped
¼ cup brown sugar
¼ cup golden syrup
395g can sweetened condensed milk
2 /3 cup custard powder
2½ cups milk
½ cup icing sugar mixture
1 Tbsp Dutch cocoa
3-4 tsp hot water
Grease an 18cm (base measurement) square cake tin with cooking oil spray. Line base and sides with baking paper, extending paper 1cm above tin. Line base of prepared tin with 9 biscuits, in a single layer, shiny side down.
Put butter, brown sugar and syrup in a medium saucepan and bring to the boil. Continue to boil vigorously, stirring constantly for 4 minutes. Remove from heat and whisk in condensed milk. Return to the stove over medium heat and cook, stirring constantly, for 8 minutes or until very thick and slightly deeper caramel in colour. Remove from heat.
Put custard powder and 250ml of the milk in a large heatproof bowl and whisk until combined, then pour in hot condensed milk mixture, whisking until smooth. Return mixture to saucepan over medium heat and pour in remaining milk. Cook, stirring constantly with a whisk, for 5 minutes or until very thick, then continue to cook, stirring vigorously for a further 30 seconds. Spoon on top of biscuits in tin. Top with a second layer of 9 biscuits, shiny side up. Refrigerate for 2 hours or until set firmly.
To make icing, put icing sugar, cocoa and water in a bowl and stir until smooth. Spoon icing over slice. Refrigerate for a further 1 hour or overnight, if convenient, until icing sets. Using biscuits as a guide, slice into 9 squares to serve.