Plum and Berry Cobbler

INGREDIENTS

• 2 3/4 pounds ripe plums, halved lengthwise, pitted, and cut into 3/4-inch-thick wedges (about 8 cups)
• 1 cup fresh blueberries, blackberries, or raspberries
• 2/3 cup granulated sugar
• 3 tablespoons cornstarch
• 2 tablespoons light-brown sugar
• 1 tablespoon fresh lemon juice
• 1 teaspoon finely grated peeled fresh ginger
• 1/2 teaspoon salt, plus a pinch
• 2 cups all-purpose flour
• 2 teaspoons baking powder
• 1/2 cup (1 stick) cold unsalted butter, cut into small pieces
• 1 vanilla bean, split lengthwise and seeds scraped (reserve pod for another use)
• 1 cup plus 2 tablespoons heavy cream, plus more for brushing
• Sanding sugar, for sprinkling