Pour melted butter into a 9"x13" dish. Add ½ pecans, coating the base.
Mix flour, cinnamon, sugar, baking powder, and salt in a bowl.
Add milk, rum, and vanilla to dry mix. Stir gently; avoid overmixing.
Spoon batter over pecans. Mix brown sugar & pecans, sprinkle over batter.
Carefully pour hot water, bake for 30-35 mins till golden.