Frozen puff pastry (2 sheets), thawed
4 3/4-inch-thick 6-ounce skinless salmon fillets
6 tablespoons minced shallots
4 tablespoons plus 2 teaspoons chopped fresh tarragon
1 egg beaten to blend (for glaze)
1/2 cup dry white wine
1/2 cup white wine vinegar
1/2 cup (1 stick) chilled butter, diced
Preheat oven to 425°F. Roll out each pastry sheet on lightly floured surface to 12-inch square. Cut each in half, forming four 12x6-inch rectangles. Place 1 salmon fillet in center of each rectangle, about 3 inches in from and parallel to 1 short edge. Sprinkle each fillet with salt, pepper, 1 tablespoon shallots and 1 tablespoon tarragon. Brush edges of rectangles with some glaze. Fold long sides of pastry over fillets. Fold short edge of pastry over fillets and roll up pastry, enclosing fillets. Seal edges of pastry. Place pastries, seam side down, on baking sheet. Brush with glaze.
Bake pastries until dough is golden brown, about 20 minutes. Remove from oven; let stand 10 minutes. Meanwhile, boil wine, vinegar and 2 tablespoons shallots in heavy small saucepan until liquid is reduced to 6 tablespoons, about 8 minutes. Remove pan from heat. Add butter 1 piece at a time, whisking until melted before adding next piece. Whisk in 2 teaspoons tarragon. Season sauce with salt and pepper.
Cut pastries into thirds. Place sauce and pastries on 4 plates.