Position racks in the upper and lower thirds of the oven; preheat to 350˚. Spread 3/4 cup pistachios on a baking sheet and bake on the lower oven rack until toasted, about 8 minutes. Let cool completely, then pulse in a food processor until finely chopped.
Line 2 baking sheets with parchment paper. Spread the granulated sugar on one of the prepared pans and bake on the lower oven rack until very hot but not melting, about 10 minutes. Reduce the oven temperature to 225˚.
Meanwhile, combine the egg whites, cream of tartar and salt in the bowl of a stand mixer fitted with the whisk attachment. About 30 seconds before the sugar comes out of the oven, beat the egg whites on medium speed until soft peaks form. Gradually beat in the hot sugar, about 1 tablespoon at a time, using the parchment paper as a funnel. Increase the mixer speed to high and beat, scraping down the bowl as needed, until stiff peaks form, 3 to 4 minutes. Add the vanilla and beat until the meringue is fluffy, about 30 seconds.
Reline the baking sheet that you used for the sugar with parchment paper. Gently fold the chopped pistachios and chocolate into the meringue with a rubber spatula until just combined. Transfer to a piping bag fitted with a 1/2-inch round tip. Pipe the meringue into 1 3/4-inch-wide mounds (about 1 1/2 inches tall) about 1 inch apart onto the prepared pans. Very finely chop the remaining 2 tablespoons pistachios, then sprinkle on the meringues.
Bake, switching the pans halfway through, until the meringues are firm to the touch and lift easily off the parchment, 40 to 50 minutes. Turn off the oven and let the meringues sit in the oven for 10 minutes, then remove the meringues to racks to cool completely.