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

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