Preheat oven to 425°F (220°C) and pour melted butter into a baking dish.Mix flour, baking powder, sugar, and salt in a bowl.Add buttermilk and vanilla, then stir until just combined.Fold in diced peaches gently.Spread the batter evenly over the melted butter.Score the dough into squares.Bake for 25–30 minutes until golden brown.Cool slightly and serve warm.