1 cup shelled unsalted pistachios 4 large egg yolks 1/2 cup plus 1 tablespoon granulated sugar 2 cups heavy cream 1/4 cup air-dried light brown sugar (see Tip)
Preheat the oven to 350°F. Place the oven rack in the middle position of the oven.
Spread the pistachios on a baking sheet and toast until the nuts are fragrant but not browned, about 4-5 minutes. Remove from the oven and lay a towel over the top. Let sit for 10 minutes. Rub off and discard and loose skins (some will remain on the nuts).
Place the nuts and granulated sugar in a food processor and pulse until finely chopped.
Place the nut mixture and cream in a 1 quart heavy saucepan and cook over medium heat, uncovered, stirring occasionally, just until hot (but not boiling). Remove from the heat, cover and let sit for 30 minutes.
Rinse a kitchen towel under cold water and wring out as much water as possible. Line a large sieve with the damp kitchen towel and set over a large bowl. Pour the pistachio mixture into the towel. Wrap the towel up and working over the sieve, squeeze the pistachio cream from the towel into the bowl. Discard the ground pistachios.
Lightly beat the egg yolks in a medium bowl. Whisk in the pistachio cream. Divide the mixture among four 5-ounce ramekins or shallow gratin dishes.
Set the ramekins in a small baking dish and add enough hot tap water to the dish to reach halfway up the sides of the ramekins. Bake for about 20-30 minutes, or until the custards are just set around the edges but the centers tremble slightly when the dishes are gently shaken. Remove the ramekins or gratin dishes from the water bath and cool, uncovered. Cover and refrigerate for at least 4 hours or overnight.
Preheat the broiler. Sift a thin even layer of the air-dried brown sugar over each of the custards. Broil one ramekin at a time as close as possible to the heat source until the brown sugar melts, forming a caramelized crust on top of the custard. Serve immediately.
To air-dry brown sugar, sift it onto a plate and leave it out, uncovered, for 1 day. Alternatively, if the weather is humid, the sifted brown sugar can also be dried in the oven. Set the temperature at 250ºF and put the sugar in the oven for 20 minutes.