Preheat oven to 350 degrees. Place the butter in a 9x13 inch pan and heat the pan in the oven to let the butter melt. In a medium sized mixing bowl combine the flour, sugar, baking powder and salt. Stir in the milk. Remove the pan from the oven and spread the batter evenly on top. Top with pecans, brown sugar, and toffee bits. Slowly pour the boiling water on top, do not stir. Bake uncovered 30-35 minutes or until the top is golden brown. Let it cool on a wire rack 20-25 minutes and the cobbler will thicken while cooling. Serve warm with ice cream on top.