While the bars are setting, prepare your praline topping.
Spread the pistachios over a lined tray or silicone baking mat.
In a small saucepan, combine brown sugar and water. Heat over medium heat until the mixture begins to bubble and turns amber in color - about 7-10 minutes. (Don't stir once it starts bubbling or the sugar may crystalize. Just swirl the pan gently)
Quickly pour the hot caramel over the pistachios, coating them well. Let cool to room temperature and harden (about 20-30 minutes)
Once set, break in to chunks and pulse in the food processor until you reach your desired texture - chunky or fine.
Cut the chocolate pistachio bars in to squares and sprinkle the pistachio pieces on them before serving