Strawberry Buckwheat Bars

Name: Strawberry Buckwheat Bars
Ingredients: 1 cup (2 sticks) cold unsalted butter, cut into pieces, plus more for pan; 2 1/4 cups all-purpose flour; 3/4 cup buckwheat flour; 1/2 cup granulated sugar; 1/2 cup light brown sugar; 1/2 teaspoon kosher salt; 1 teaspoon baking powder; 1 large egg, beaten to blend; 3 cups hulled, quartered strawberries (about 1 pound); 1 (12–13-ounce) jar strawberry preserves; 1 tablespoon cornstarch; 2 teaspoons fresh lemon juice; Powdered sugar (for dusting; optional)