In a bowl combine 1 cup brown sugar,flour,3 tablespoons cocoa,baking powder and salt. In another bowl combine milk,melted butter and vanilla extract. Stir into dry gredients just until combined.
Spread evenly in a slow cooker coated with cooking soray.Sprinkle with chocolate chips.
In a 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 hours or until a tooth pick inserted near center comes out clean.Serve warm with ice cream.