Pour melted butter into a 9x13-inch baking dish. Set aside.
In a medium bowl, whisk flour, sugar, baking powder, and salt together.
Make a well in the center.
Add milk and vanilla extract, and whisk until just combined. Batter will be on the thicker side.
Add batter to pan.****
Sprinkle with brown sugar and then pecans.
Pour hot water over the top. DO NOT STIR.
Bake for 30-40 minutes, or until the top looks set. You will see the pudding bubbling up in spots, and that's ok.
Let sit 10 minutes before serving. The pudding portion will continue to thicken as it sits.
*Be sure to aerate (stir), spoon, and sweep when measuring your flour to avoid packing in extra flour. **Anything from skim to whole milk will work. ***Hot tap water is fine. ****Since the batter is thick, it's easier to drop dollops of batter over the bottom of the pan and then gently spread them together. It's ok if the butter shows in spots or around the edges. *****Nutrition values are estimates.