Heat your oven to 350°F. Place butter in a 2 to 3-quart (3 was recommended, but I had a lot of extra space) baking dish or a 7×11-inch cake pan. Place pan in oven for 5 to 7 minutes, until butter has melted. Tilt the dish to ensure it is evenly coated with butter.
Whisk together the flour, 3/4 cup sugar, baking powder, and salt in a small bowl and stir in milk. Mix until smooth. Pour batter over the butter but do not stir, even if it looks like a puddle-y buttery mess.
Cut apples into 8 wedges, small plums into 4 wedges and medium ones into 4 to 6. Space fruit over batter. Sprinkle with cinnamon and then remaining sugar.
Bake the cobbler until it is set and golden on top, 35 to 40 minutes. Cool slightly on a rack. Serve the cobbler warm or at room temperature.