Use a little of the butter to grease the base and sides of a 20 cm (8 inch) round spring-form tin. Put the rest of the butter in a saucepan with the condensed milk and chocolate. Heat gently for 3–4 minutes, stirring until melted, then remove from the heat.
Place the biscuits in a plastic bag and crush roughly into chunky pieces using a rolling pin. Toast the hazelnuts under a preheated hot grill until lightly browned, then roughly chop with the pistachios.
Stir the biscuits into the chocolate mixture then spoon half the mixture into the prepared tin and spread level. Reserve 2 tablespoons of the nuts for the top, then sprinkle the rest over the chocolate biscuit layer. Cover with the remaining chocolate mixture, level the surface with the back of the spoon and sprinkle with the reserved nuts.
Chill the nut mixture for 3–4 hours until firm, then loosen the edges and remove the sides of the tin. Cut into 10 thin slices, or into tiny bite-sized pieces to make petits fours. Store any leftovers in the refrigerator, wrapped in foil, for up to 3 days.