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)