Turn your 3-quart crockpot on high. Coat the inside of the crock with non-stick spray.
In a small bowl, combine 1 cup brown sugar, flour, 3 tablespoons cocoa, espresso powder, baking powder and salt. In a large bowl, combine the milk, butter and vanilla. Add the dry ingredients to the wet ingredients and mix to combine. Spread the mixture evenly into the prepared slow cooker. Sprinkle with chocolate chips.
In another small bowl, combine the remaining brown sugar and cocoa; stir in the boiling water. Pour the hot water mixture over the batter but do not stir it in with the already made cake mix. The water is supposed to sit on top of the cake batter. Cover and cook on high for 2 to 2-1/2 hours or until a toothpick inserted near the center of the cake comes out clean. Serve warm with ice cream and hot fudge.