Preheat oven to 350°F.
Pour melted butter into the bottom of a 9×13-inch baking dish. Do not stir.
In a bowl, whisk flour, granulated sugar, baking powder, and salt.
Stir in milk and vanilla extract until just combined.
Pour the batter evenly over the melted butter in the baking dish. Do not stir.
Sprinkle chopped pecans evenly over the batter.
Sprinkle the brown sugar evenly over the pecans.
Slowly pour the hot water over the entire dish. Do not stir.
Bake 45–55 minutes or until golden brown on top and the center is mostly set.
Let cool at least 15 minutes before serving to allow the caramel layer to thicken.
Serve warm, optionally with vanilla ice cream.