Preheat oven to 350°. Spray a 13×9-inch rimmed baking sheet with baking spray with flour.
In a medium bowl, whisk together flour, ¼ cup sugar, baking powder, and salt. Using a pastry blender, cut in cold butter until mixture is crumbly.
In a small bowl, whisk together milk, egg, and vanilla. Add milk mixture to flour mixture, stirring until combined. Drop mixture by spoonfuls onto prepared pan, and spread evenly using well-oiled hands. Place plums in three rows on top of crust.
In another small bowl, stir together strawberry preserves, melted butter, and remaining ¼ cup sugar. Spread preserves mixture onto plums.
Bake until crust is golden brown, about 40 minutes. Serve warm or at room temperature.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/plum-biscuit-tart/