To make the Biscuit Base, place the butter, cocoa and sugar in a medium saucepan and cook over a low heat, stirring, until they melt together. Leave to cool slightly
Whisk in the egg, then mix in the biscuit crumbs, coconut and walnuts.
Press into the prepared slice tin and place in the freezer to firm up.
To make the custard layer, beat the butter in a large bowl until light and fluffy.
Beat in the cream and custard powder.
Slowly beat in the icing sugar a little at a time until very thick.
Spread the custard filling over the base layer and refrigerate for an hour.
To make the chocolate top layer, break the chocolate into a microwave-safe dish and microwave for 1 minute. Stir, then microwave in 20-second bursts, stirring between bursts, until melted and smooth. Stir in the oil.
Pour over the slice and smooth the top. Refrigerate until set.
Cut into 16 pieces using a hot knife (this will stop the chocolate from cracking).
A Table at Tangleby
In this exceptional book, it shares more than 170 delicious dishes, from simple snacks to dinner parties that are guaranteed to impress your guests.