¼ cup water
1 ½ tsp. cornstarch
1/3 cup plus ½ cup sugar
2 cup chopped fresh rhubarb stalks (10 oz.)
1 cup chopped fresh strawberries (5 oz.)
1 cup all-purpose flour
1 3/4 tsp. baking powder
½ tsp. salt
1 large egg
½ cup whole milk
½ cup unsalted butter, melted and cooled slightly
1 tsp. pure vanilla extract
Put oven rack in middle position and preheat oven to 400 degrees. Butter an 8-in. square glass or ceramic baking dish. Stir together water, cornstarch, and 1/3 cup sugar in a small saucepan; stir in rhubarb. Bring to a simmer, stirring constantly; simmer, stirring occasionally, for 3 mins. Remove from heat and stir in strawberries. Whisk together flour, baking powder, salt, and remaining 1/2 cup sugar in a bowl. Whisk together egg, milk, butter, and vanilla in a large bowl; whisk in flour mixture until just combined.
Reserve 1/2 cup fruit mixture, then add remainder to baking dish and pour batter over it, spreading evenly. Drizzle reserved 1/2 cup fruit mixture over batter. Bake until a wooden pick inserted into center of cake portion comes out clean, 25 to 30 mins. Cool in pan on a rack 5 mins. before serving. Serves 6-8.
