Preheat oven to 350°. Place butter in a 13x9-in. 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.