Combine flour, one cup brown sugar, 3 tablespoons cocoa, baking powder and salt in a medium bowl. Whisk in milk, butter and vanilla. Spread evenly in a slow cooker. Mix together remaining 3/4 cup brown sugar and 1/4 cup cocoa; sprinkle evenly over top of batter. Pour in hot water; do not stir. Cover and cook on high setting for 2 hours or until a toothpick inserted one inch deep comes out clean. Spoon warm cake into bowls; top with vanilla ice cream, if desired.