Pour the sugar in a wide heavy-bottomed skillet and set it over medium heat. Line a large rimmed cookie sheet with parchment paper, or a silicon mat and place it next to your stove.
Once the sugar starts to melt and clump, use a wooden spoon to stir the sugar as it cooks until the sugar has completely melted, and is a deep golden color.Note: the sugar changes its color as the temperature rises, at a certain temperature (350F/180c), when it is a deep amber color it get bitter. You do not need a thermometer (but you can use one), just pay attention and constantly stir, as it helps moderate the process.
Pour the liquid hot caramel (do not touch it with your bare fingers!) over the prepared lined pan, and allow it to naturaly spread.
Sprinkle the pistachios over the surface of the caramel, then set is aside for 15-20 minutes to set and re-crystalize.
Once set, break the praline (this is a great snack) and place it in the food processor. Add the vanilla and salt if using.
Process for 3-5 minutes, scraping the bowl 2-3 times until the mixture is thin and spreadable consistency. Note: as you process, it will go from a powder consistency, to a dough consistency (play dough), to a thin paste like consistency. You can stop and use it at any stage.
Scrape the spread into an airtight container, use or refrigerate.
Notes
Store in an airtight container for up to 7 days at room temperature, or refrigerate for up to 14 days.
Depending on your food processor, it takes up to 20 minutes for the praline to reach a smooth, Nutella spread consistency. To make it thinner, add 2-4 teaspoons of unflavored oil like Canola oil.