Adjust an oven rack to be about 5 inches away from the broiler heat element and preheat the broiler to high.
Combine the cardamom, brown sugar, and salt in a small ramekin.
Place the nectarines cut-side up in a single layer on a baking sheet.
Dot the nectarines with the butter and sprinkle with the brown sugar mixture.
Broil for 7 to 10 minutes, until golden brown and caramelized, basting with the juices from the baking sheet halfway through using a spoon or heat-resistant brush.
Remove from the oven.
Pour the liquid from the baking sheet into a measuring cup and reserve for the caramel sauce; set the nectarines aside to cool.
Add enough butter to the pan juices reserved from the nectarines to make 6 tablespoons total. Transfer to a small bowl and set aside.
To assemble the sundae, place 2 generous scoops of vanilla ice cream in a bowl, top with the roasted nectarines, and garnish with crushed pistachio brittle.
Drizzle with the caramel sauce and serve.
