Preheat oven to 350°.
Place butter in a 13x9" baking pan; heat pan in oven 3-5 minutes or until butter is melted.
Meanwhile, combine the flour, sugar, baking powder and salt.
Stir in milk and vanilla until combined.
Remove baking pan from oven; add batter.
Sprinkle with pecans, brown sugar and toffee bits.
Slowly pour boiling water over top (do not stir).
Bake, uncovered, until golden brown, 30-35 minutes.
Cool on wire rack for 30 minutes (cobbler will thicken upon cooling).
Serve warm, with ice cream if desired.