Heat the oven to 350°F and place the butter in a 9-by-13-inch baking dish.
Set the dish in the oven for 5 to 7 minutes until the butter melts, then remove it carefully.
Whisk the flour, granulated sugar, baking powder and salt, then whisk in the milk and vanilla until mostly smooth.
Pour the batter evenly over the butter without stirring the layers together.
Arrange the drained peaches over the batter, sprinkle with brown sugar and cinnamon and again do not stir.
Bake for 45 to 50 minutes until the crust rises around the fruit and turns deep golden; rest 20 minutes.