Preheat oven to 350°. Prepare a baking dish (8” square, or similar) with a tablespoon of melted butter spread evenly inside. Combine fruit (hold a few sugared cherries aside to garnish top), lemon juice and brown sugar, spreading evenly over bottom of baking dish. Combine flour, salt and baking powder. Add butter, vanilla, lemon zest, sugar and milk, and stir well with wooden spoon or spatula. Pour over fruit, spreading gently (it should have thick and thin spots where fruit pokes through). Scatter reserved cherries over the top for garnish. Mix corn starch and a tablespoon of cold water. Into rapidly boiling water, stir sugar and corn starch slurry and whisk until smooth and thickened slightly. Pour over cake. Bake until water is absorbed, center is puffed and golden and fruit is bubbling. Serve immediately with ice cream or whipped cream, as desired.