Mousse: Line an 8 1/2" X 4 1/2" loaf pan with plastic wrap, leaving a generous overhang on all sides; smooth plastic to avoid lines in finished mousse.
Place 1 tablespoons water in a small bowl. Sprinkle gelatin over and let sit 5 minutes.
Using electric mixer on high speed, beat egg yolks and salt in a large bowl until pale yellow and frothy, about 4 minutes.
Fit a small saucepan with candy thermometer and bring corn syrup, sugar, and 1/4 cup water to a boil. Cook until thermometer registers 238 degrees. Remove from heat and add gelatin, stirring to dissolve with mixer on medium-low, drizzle syrup into egg mixture. Mix in chocolate.
Using electric mixer with clean beaters, beat cream in a medium bowl to soft peaks. Mix one-third whipped cream into egg mixture. Fold in pistachios, then remaining whipped cream. Scrape mousse into prepared pan; cover tightly with plastic wrap. Freeze until solid, at least 5 hours.
Sour Cherry Sauce: Combine cherries and 2 cups very hot water in a small bowl; let sit 10 minutes to soften.
Meanwhile, bring cherry juice, sugar, and 1/2 cup water to a boil in a small saucepan over medium-high heat. Reduce heat. Drain cherries and add to saucepan; simmer until sugar is dissolved and mixture is slightly thickened, 8-12 minutes. Let cool. Can be made 3 days ahead. Cover and chill. Bring to room temperature before using.
Topping and Assembly: Combine cream, sour cream, sugar, and salt in a medium bowl. Scrape seed from vanilla bean; discard pod. Beat to soft peaks.
Let mousse thaw 5-10 minutes, then unmold, unwrap, and slice with a hot knife. Serve with cherry sauce and topping. Place mousse slice ON TOP of Cherry Sauce on dessert plate and topping on the side the mousse.
Slice mousse into portions and freeze on a parchment-lined baking sheet the day of the party to make serving easy.