Preheat your oven to 350°F.
Place the butter in a 9x13-inch baking dish or casserole pan and melt it in the oven.
Once the butter has melted, sprinkle the pecans evenly over the butter.
In a mixing bowl, combine the flour, sugar, cinnamon, milk, and vanilla. Stir gently until just combined, being careful not to over-mix.
Pour the batter over the melted butter and pecans without stirring.
Evenly sprinkle the brown sugar over the batter, again without stirring.
Carefully pour the hot water over the top, making sure not to mix it in.
Bake for 30-35 minutes, or until the cobbler is golden brown and bubbling. Enjoy!