Put the hazelnuts in a food processor and process until fairly finely chopped. Scoop out 3 tablespoons of the hazelnuts, then whizz the nuts in the processor until the consistency of ground almonds. Add the polenta, caster sugar and butter and blend to a paste.
Pack into a greased and base-lined 18 cm (7 inch) sandwich tin (see
Beat together the icing sugar and a dash of water in a bowl to make a smooth, spoonable icing, then drizzle over the biscuit and cut into wedges.