Instructions

  1. Put oven rack in middle position and preheat oven to 400°F
  2. Butter an 8-inch square glass or ceramic baking dish.
  3. Stir together water, cornstarch, and 1/3 cup sugar in a small saucepan, then stir in rhubarb.
  4. Bring to a simmer, stirring constantly, then simmer, stirring occasionally, 3 minutes. Remove from heat and stir in strawberries.
  5. Whisk together flour, baking powder, salt, and remaining 1/2 cup sugar in a bowl.
  6. Whisk together egg, milk, butter, and vanilla in a large bowl, then whisk in flour mixture until just combined.
  7. Reserve 1/2 cup fruit mixture, then add remainder to baking dish and pour batter over it, spreading evenly.
  8. Drizzle reserved 1/2 cup fruit mixture over batter.
  9. Bake until a wooden pick inserted into center of cake portion comes out clean, 25 to 30 minutes.
  10. Cool in pan on a rack 5 minutes before serving.