Preheat the oven to 350°F.

In a large mixing bowl, combine the melted butter, flour, granulated sugar, baking powder, milk, and vanilla extract. Stir until smooth, about 1-2 minutes.

Transfer the smooth batter into a greased baking dish, spreading it evenly across the bottom.

Arrange the sliced peaches on top of the batter.

In a small bowl, mix the brown sugar with a squeeze of fresh lemon juice and sprinkle it over the peaches.

Bake for about 45 minutes, or until golden brown and a toothpick comes out clean.