Lightly grease and line a 15cm x 24cm dish or cake pan.
Crush the biscuits in a food processor — but not too finely.
Melt the butter and sugar in a saucepan over low heat. Whisk in the egg and bring to boiling point. Add the cocoa, walnuts, sultanas and vanilla essence. Press into the prepared pan and chill.
Meanwhile, sift the icing sugar and cocoa into a bowl. Melt the butter in the boiling water and stir into the icing sugar with the vanilla. Beat well.
Spread the chilled slice with the icing. Best kept for a day before slicing into squares or oblongs. Makes about 21 pieces.
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