Preheat the oven to 400˚F. Grease a 9×13-inch baking pan with butter and set aside.
Sift the flour, sugar, baking powder, and salt into a large bowl or the bowl of a standing mixer. With an electric hand mixer or standing mixer fitted with a paddle attachment, beat in the butter until a crumbly mixture forms. In a separate small bowl, mix together the egg and milk until combined. Add to the crumbly mixture and beat on low speed until evenly combined. Transfer the batter to the greased baking pan and smooth the surface with a spatula. Top with the halved prunes, cut sides up.
In another small bowl, combine the sugar, nutmeg and cinnamon. Sprinkle all over the surface of the batter. Drizzle the melted butter over top. Bake for about 25 minutes or until a toothpick inserted comes out clean.
Transfer to a wire rack to cool before serving.