Cut butter into 1/2-in. cubes and freeze in a single layer until very cold, 10 to 20 minutes. In a food processor, whirl flour and salt until blended. Add butter and pulse on and off just until bits of butter are about the size of peas. In a small bowl, lightly beat egg and yolk with 3 tbsp. ice water. Add egg mixture, about half at a time, and pulse just until evenly moistened and holding together (you may need to add 1 more tbsp. ice water as you're pulsing).
Flatten the dough into 2 rough squares, each about 6 in. wide. Wrap squares in wax paper. Chill until cold, at least 1 hour and up to 3 days.
Preheat oven to 350°. Remove dough from refrigerator and let stand 5 minutes. Unwrap and, on a lightly floured surface, roll dough halves into 1/8-in.-thick squares, 10 to 11 in. each.
Transfer each square to a rimmed baking sheet and prick all over with a fork. Bake until medium brown, 25 to 35 minutes. Cool on pan, about 30 minutes. Spread with crème fraîche, dividing evenly, and top with smoked salmon and capers. Cut into 2-in. squares.
Note: Nutritional analysis is per 2-piece serving.