Combine first 5 ingredients in a 9-inch square baking pan, and stir well. Add milk, oil, and vanilla, stirring until smooth. Stir in chocolate morsels. Combine brown sugar and 1/4 cup cocoa; sprinkle over batter. Combine water and coffee mix, stirring to dissolve. Pour coffee mixture over batter; do not stir. Bake at 350 degrees F for 40 minutes or until cake springs back when touched lightly in center. Serve warm with 1 tablespoon frozen yogurt.