Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 5 ingredients in a bowl. Add milk, oil, and vanilla, stirring just until combined.
Pour batter into an 8-inch square baking pan coated with cooking spray. Combine brown sugar and 2 tablespoons cocoa in a small bowl; sprinkle evenly over batter. Pour hot water over batter (do not stir).
Bake at 350° for 35 minutes. Remove from oven; sprinkle with toffee bits. Let stand 15 minutes. Serve warm.