Preheat oven to 375 degrees F. Have ready an 8-inch square baking dish. No need to grease it.
In a large bowl, toss the berries with the sugar and cinnamon and let stand for 20 minutes. A syrup should form.
In a mixing bowl, mix together flour, baking powder, salt and milk, then stir in melted butter.
Spread the batter in the pan, the pan, then spoon the berries in the center.
Bake for 45 to 55 minutes or until batter is golden brown.
Notes
You can cut the sugar by a tiny bit if you want to. I usually use a scant 1 cup which is about 180 grams or 1 cup minus about 2 tablespoons. 1 cup seems like a lot, but with all of the peaches and butter, it ends up not being overly sweet. But I still like to use just shy of a cup.