Put the sugar and cream in a heavy-based saucepan and heat gently, stirring occasionally, until the sugar has completely dissolved. Stir the butter into the sugar and cream mixture, insert a sugar thermometer if you have one, and increase the heat. Cook for about 15 minutes until the temperature reaches 120°C (250°F) or a little of the toffee forms a hard ball when dropped into a bowl of cold water. Stir occasionally at first, then more frequently as the mixture begins to colour, froth and thicken. When the butter begins to separate out of the toffee, it is almost ready. Cook for a few more minutes until it changes to a smooth, glossy toffee.
Remove from the heat, quickly stir in the pistachio nuts, then pour the toffee into a greased 18 cm (7 inch) shallow square cake tin. Allow to cool for 5 minutes, then mark into strips and leave to cool completely.
Loosen the edges of the toffee from the sides of the tin and turn it out. Break into pieces with a toffee hammer or by hitting a cook's knife with a rolling pin. The toffee will keep, in an airtight container in a cool place, for up to 1 week.