Step 1Cut a sheet of wax paper, parchment paper or aluminum foil that is long enough to fit around a 1-quart souffle dish and fold it in lengthwise quarters to make a ribbon about 3 inches wide. Wrap this around the top of the dish to make a collar and tape the ends together. The collar should extend about 2 inches above the rim of the souffle dish. Secure in place with a rubber band and place the dish in the freezer to chill.
Step 2Zest enough tangerines (about 3) with a fine grater to make about 1 teaspoon. Put the zest in a small saucepan and juice all of the tangerines over it (you should have about one-half cup of juice). Warm the juice over low heat, add the gelatin and stir until it dissolves completely. Remove the pan from the heat and stir in the vanilla extract. Place the pan in a bowl of ice water and stir until the mixture cools and thickens to a jelly.
Step 3Bring a small stock pot of water to a simmer. Beat together the egg yolks and sugar in a mixing bowl and place the bowl 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 quite pale and have approximately doubled in volume. When you rub a little of the egg yolk mixture between your fingers, you shouldn't feel any grit from the sugar.
Step 4Remove the bowl from the heat, whisk in the thickened tangerine juice and continue whisking until the mixture has cooled, a few minutes. It should be thick and sticky.
Step 5Beat the egg whites to stiff peaks, using a standing mixer, or in a medium bowl using a hand mixer. Separately, beat the whipping cream to soft peaks. Spoon about half of the egg whites into the cooled egg yolk mixture and stir to lighten the mixture. Add the remaining egg whites to the whipped cream and pour the tangerine mixture on top. Gently fold them all together to make a uniform mixture.
Step 6Gently spoon the mixture into the souffle dish. It should come at least halfway up the paper collar. Smooth the top and freeze until very firm, at least 4 hours. If you freeze it overnight, transfer it to the refrigerator for at least 1 hour to soften before serving. If you make it more than 1 day in advance, once the mixture is frozen solid, cover it with plastic wrap and gently press the wrap against the surface of the souffle. When ready to serve, sprinkle the top with the chopped pistachios, and carefully remove the collar.