Preheat oven to 350°F (175°C). Place the melted butter into a 9x13-inch baking dish.
In a medium bowl, whisk together flour, sugar, baking powder, and salt. Stir in the milk until just combined.
Pour the batter evenly over the butter in the baking dish. Do not stir.
In another bowl, toss the peaches with lemon juice and cinnamon.
Spoon the peach mixture evenly over the batter. Again, do not stir.
Bake for 40–45 minutes or until the top is golden brown and the batter is cooked through.
Let cool slightly before serving.