Tip half the chocolate into a bowl set over a pan of simmering water. Once melted, remove from the heat and stir in the rest of the chocolate. Put the petit four cases into the holes of the mini muffin tins. Put 1 tsp of the melted chocolate in each one, then pick up a petit four case in your hand and swirl it gently to get the chocolate to coat the sides. Repeat with all the cases, leaving about a third of the chocolate in the bowl. Pop the tins in the fridge to firm up while you make the fillings.
Put the butter in a large bowl with the icing sugar. Beat well with electric beaters or a wooden spoon until light and creamy. Divide the mixture into three bowls, then stir the peanut butter into one, hazelnut butter into the second and pistachio butter into the third (if using). Transfer the fillings to three piping bags.
Take the chocolate cups out of the fridge and fill with the nut mixtures. If your reserved chocolate has hardened, melt again as before. Once melted, carefully spoon it over the top of each cup to seal in the nut butters.
Top each cup with with a whole nut (or a nut coated in gold leaf, a pinch of coarse sea salt – or any combination). Chill for at least 2 hrs. Will keep in the fridge for 1 week (see tip).