Blackberry-Raspberry Hand Pies

INGREDIENTS

• 1 tablespoon sour cream
• 1 tablespoon fresh lemon juice (from 1 lemon)
• 2 tablespoons ice water, plus more if necessary
• 1 1/4 cups all-purpose flour, plus more for rolling
• Coarse salt
• 1/3 cup plus 1 teaspoon granulated sugar, divided
• 1 stick cold unsalted butter, cut into small pieces
• 3/4 cup fresh blackberries
• 3/4 cup fresh raspberries
• 1 tablespoon plus 1 1/2 teaspoons cornstarch
• 1 large egg, lightly beaten
• Fine sanding sugar, for sprinkling