Plum Galette with Cornmeal Crust

Name: Plum Galette with Cornmeal Crust
Ingredients: 1 cup all-purpose flour, plus more for dusting; ¼ cup fine cornmeal; 1 tablespoon natural cane sugar; ½ teaspoon coarse salt; 1 stick (1/2 cup) cold unsalted butter, cut into small pieces; 1 large egg yolk; 2 to 4 tablespoons ice water; ¼ cup natural cane sugar; 1 vanilla bean, split and seeds scraped; 2 tablespoons cornstarch; 1 ¼ pounds small plums, pitted and sliced into ½-inch wedges; 1 tablespoon unsalted butter, cut into pieces; 1 large egg lightly beaten with 1 teaspoon water; Raw sugar, such as turbinado, for sprinkling