In a small bowl, combine 1 cup brown sugar, flour,3 tablespoons cocoa, baking powder and salt. In another bowl, combine the milk, butter and vanilla; stir into dry ingredients just until combined. Spread evenly into 3 quart slow cooker coated with cooking spray. Sprinkle with chocolate chips.
In another small bowl, combine the remaining brown sugar and cocoa: stir in boiling water. Pour over batter (do not stir) Cover and cook on high for 4 to 4 1/2 hours or until a toothpick inserted near the center of cake comes out clean Serve warm with ice cream.