1 ½ x 200g packets lattice biscuits
½ cup vanilla-flavoured custard powder
¼ cup cocoa powder, sifted
½ cup caster sugar
1 litre (4 cups) milk
300ml tub thickened cream
100g dark chocolate, finely chopped
2 teaspoons vanilla extract
5 teaspoons powdered gelatine
200g dark chocolate, finely chopped
1/3 cup thickened cream
Grease a 24cm square cake pan (base measures 20.5cm, with slopping sides). Line base and sides with baking paper, extending paper 3cm above pan edges.
Arrange 13 of the biscuits, glazed-side down and trimming to fit, over base of pan.
Combine custard powder, cocoa and sugar in a large saucepan. Gradually whisk in ½ cup of the milk until smooth. Add remaining milk and cream. Whisk, over a high heat for about 8 to 9 minutes, or until mixture boils and thickens. Continue whisking for a further 2 minutes.
Remove pan from heat. Add chocolate and vanilla, stir until melted and smooth.
Sprinkle gelatine over 1/3 cup water in a small heatproof jug. Sit jug in a saucepan of simmering water. Stir until gelatine is dissolved. Whisk into hot chocolate custard.
Pour over biscuits in pan. Place 16 of the biscuits, glazed-side up over top, reserving remaining for another use. Refrigerate overnight.
To make chocolate ganache, combine chocolate and cream in a microwave-safe bowl. Microwave on High (100%) in 20 second bursts, stirring until smooth. Pour over slice. Spread evenly. Refrigerate until set.
To serve, lift slice from pan. Using a hot knife, cut into squares.