To form a collar, cut a sheet of waxed paper, aluminum foil or parchment that's long enough to fit around a 1qt souffle dish. Fold it in quarters lengthwise to make a ribbon about 3" wide. Wrap the collar around the top of the souffle, and tape the ends together. The collar should extend about 2" above the rim of the dish; secure with tape and/or a rubber band, and place in the freezer to chill.
Zest the tangerines with a fine grater/microplane to make about 1t zest (or more to taste) and juice the tangerines, until you have 1/2c of juice. In a small saucepan, combine the zest and juice, and warm over low heat. Add gelatin and stir until it dissolves, then add vanilla extract. Place the saucepan in a bowl of cold water until the mixture thickens into a jelly (or place in a bowl of ice water if your kitchen is warm, or if you live in a hot climate). Set aside.
Bring a small stockpot of water to a simmer. Mix egg yolks & sugar in a medium metal or glass mixing bowl, and place over the simmering water (being careful not to let the bottom of the bowl touch the water). Immediately start whisking the egg yolks and continue until they are pale, and have approximately doubled in volume. You'll know the sugar has melted when you rub a little of the mixture between your fingers, and you don't feel any grit from the sugar.
When the yolks have doubled in volume, whisk in the thickened, cooled tangerine juice, and continue whisking and scraping until the entire mixture has cooled further (a few minutes). It should be thick and sticky. Set aside.
In a medium bowl, beat egg whites to stiff peaks; set aside (I stowed them in the fridge). In a large bowl, beat heavy cream to soft peaks; set aside. Spoon half of the egg whites into the cooled egg yolk mixture, and stir until its color becomes lighter. Add remaining egg whites to the large bowl of whipped cream, and add the tangerine mixture on top. Gently fold them all together to make a uniform mixture.
Gently spoon the mixture into the chilled souffle dish. It should come about halfway up the paper collar, but if not, it will still taste and look good (my dish was slightly bigger, and it turned out great). Freeze until firm, about 4hrs. If you freeze it overnight, transfer it to the fridge for at least 1hr so it can soften before serving. However, ours froze for six hours, and 20 minutes in the fridge wasn't enough for it to thaw as much as we would have liked (so use your best judgement).
When ready to serve, if desired, sprinkle with pistachios, and carefully remove the collar. Cut into sectons with a very sharp knife, and carefully remove with a small offset spatula.
If you make this in advance and it will be in the freezer for more than 4hrs, once it freezes, cover it with plastic wrap and gently press the wrap against the surface of the dish.