Line the back of a half sheet pan with a silicon mat (such as silpaor parchment paper (if using parchment paper, first spray the surface with non-stick spray — this will help the sheet to adhere to the tray as you pour the caramel). Lay the pan flat on a stable surface (lined side up).

Heat sugar, syrup, salt and water in a medium-sized heavy-bottomed pot over medium heat. Stir until the sugar dissolves, and insert the candy thermometer. Leave mixture to simmer without disturbing until the temperature reaches 300°F (hard crack stage). The mixture should turn a light amber color.

Remove pot from heat and place on a stable, heat-proof surface. Working quickly, stir in butter, vanilla, baking soda and 1/2 cup of the pistachios. Mixture will foam up - keep stirring. Beginning at one end of the lined pan, pour the caramel evenly onto the mat or paper. Use an offset spatula to smooth it into a solid sheet. Work quickly — the caramel will begin to hard in a matter of minutes. Let stand and cool for at least 15 minutes.

While the caramel layer is cooling, make the ganache. Heat a small pot of water to a gentle simmer, then place a heat-proof mixing bowl on top of the pot (or use a double boiler). Add the cream and allow to heat (cream will begin to foam or swirl). Add the chocolate, heat briefly, then stir until chocolate and cream are thoroughly combined, smooth and creamy. Pour the chocolate over the caramel layer and use an offset spatula to spread and smooth out.

Sprinkle the finishing salt and remaining 1 cup pistachios over the chocolate and use the flat of your hand to gently press the nuts into the chocolate. Set the pan aside to cool and set completely. (For a faster result, place the pan in the fridge, or outside if the weather is dry and cold.)

To break apart, gently bend up one end of the mat or parchment paper while pressing down on the center of the layer to create the first crack. From there, it should be easy to break by hand. Store in an airtight container.