Line a baking sheet with parchment paper, spray parchment well with non stick spray
In a medium large saucepan heat butter, sugar and water. Bring to a boil and stir until sugar dissoves. Cook, on a slow boil , swirling pan and stirring occasionally to cook evenly until mixture reaches 280 on a candy thermometer and is golden
Remove from heat and stir in vanilla and salt. Very carefully pour mixture onto prepared pan, smoothing to a even 1/4 inch thickness with a off set spatula
Sprinkle with 1/2 of the chopped chocolate, let it sit one minute then spread in an even layer with off set spatula
While chocolate is warm sprinkle 1/2 of the pistachios pressing lightly so they stick
Place toffee in refigerator to harden, at least 4 hours or overnight
Remove from refigerator and carefully turn toffee over on a clean parchment lined baking pan
Melt remaining chocolate in microwave or over a double boiler until smooth. Working quickly pour chocolate over flipped side of toffee spreading evenly and sprinkling with remaining pistachips, pressing to stick. Return to refigerator to set about 1 hour
Break toffee into pieces. Keep refrigerated. This can also be frozen and taken out as needed
NOTE, For easy clean up of the sticky pans fill them with water and bring to a simmer. Then just pour hot water out, the sticky toffee will have melted off!
