Preheat oven to 400 degrees F (200 degrees C).
Grease an 8 inch square pan with butter and dust with flour. Set aside.
Pit, and cut the Plum (1 pound) into eighths. Mash the Banana (1/4 cup). Cut the chilled Butter (4 tablespoon) into 1/2 inch pieces.
Whisk together All-Purpose Flour (1 cup), Baking Powder (1 teaspoon), Granulated Sugar (1/2 cup), and Salt (1/4 teaspoon) in a medium bowl. Add chilled butter to the flour mixture, and work with a pastry blender.
Mix together Milk (6 tablespoon), Vanilla Extract (1/2 teaspoon), and banana in a small bowl. Add mixture to the dry ingredients, and stir with a wooden spoon until just combined. Transfer to the prepared baking dish, and spread evenly.
Arrange plums in rows on top of dough. Squeeze in as many plums as possible. Combine Granulated Sugar (2 tablespoon) and Ground Cinnamon (1/2 teaspoon) and then sprinkle over the plums. Dab with remaining chilled Butter (3 tablespoon)
Bake until browned and bubbly, about 30 minutes. Let cool slightly before serving with Whipped Cream (to taste) and Pistachios (to taste). Enjoy!