In a small bowl, combine 1 cup brown sugar, flour, 3 tablespoons cocoa, baking powder and salt. In a second bowl, combine the milk, butter and vanilla; stir into dry ingredients just until combined.
Spread into a 3-qt. slow cooker coated with cooking spray. Sprinkle with chocolate chips. In another bowl, combine the remaining ¾ cup brown sugar and 3 Tbsp. cocoa; stir in boiling water. Pour over batter (do not stir).
Cover and cook on high for 4 to 4-½ hours or until a toothpick inserted in the center of cake comes out clean. Serve warm, with ice cream.
