Preheat the oven to 375°F. Line a baking sheet with foil and then cover foil with parchment paper. Place matzo on top of parchment, filling the baking sheet. Break extra pieces to fit any spaces.
Combine butter, brown sugar, and salt in a large saucepan and cook over medium heat.
Stir constantly until mixture comes to a boil, then continue to stir for 3 minutes.
Remove mixture from heat and pour evenly over all the matzo crackers. Place baking sheet in the oven for 15 minutes and reduce heat to 350°F. Watch carefully. If caramel seems to be browning too quickly, lower heat to 325°F.
Remove from oven and immediately sprinkle with chopped chocolate. Let stand for 5 minutes to allow chocolate to melt, then gently spread over the caramel mixture.
If desired, sprinkle chopped pistachios over crackers.
Freeze for at least 30 minutes and then break into pieces before serving.