1.Preheat oven to 180°C. Grease a 20cm x 30cm slice pan; line base and long sides with baking paper, extending the paper 5cm over sides.
2.Process biscuits until finely crushed. Combine 1 cup of the biscuit crumbs with butter in a medium bowl, then press crumb over the base of the prepared pan. Refrigerate for 10 minutes.
3.Beat egg whites in a small bowl with an electric mixer until soft peaks form. Gradually add caster sugar, beating until dissolved after each addition; fold in hazelnut meal, remaining biscuit crumbs and sifted flour. Spread mixture over biscuit base.
4.Bake 20 minutes or until firm; cool 20 minutes. Reduce oven to 160°C (140°C fan-forced).
5.For the topping, beat butter, sugar, egg yolks and liqueur (or juice) in a small bowl with an electric mixer until the sugar is dissolved, then fold in melted chocolate.
6.Spread topping over slice and bake for about 20 minutes or until set. Cool in pan. Refrigerate until firm.
7.Just before serving, dust slice with sifted cocoa; top slice with random strips of baking paper, dust with icing sugar, then carefully remove paper. Cut slice into 24 pieces.
This recipe can be made a week ahead.
You can also use stencils to dust the design onto the slice. This slice can be stored in an airtight container in the fridge for up to 1 week.
Buy plain chocolate biscuits for this recipe, that is, without filling, icing or chocolate coating.
Separate the eggs; use the egg whites in the biscuit base and the egg yolks in the chocolate topping.