In a large bowl, combine the first four ingredients. Stir in oats and coconut. Add butter and milk. Press into a greased 13-in. x 9-in. baking pan. Bake at 350° for 10-12 minutes. Remove from the oven. Reduce heat to 325°.
In a small saucepan, melt the butter and chocolate; cool slightly. In a large bowl, beat the eggs, brown sugar, milk and vanilla until well blended. Beat in the chocolate mixture. Combine the flour, baking powder and salt; gradually add to chocolate mixture. Stir in walnuts. Spread evenly over crust.
Bake at 325° for 25-30 minutes or until a toothpick comes out with moist crumbs (do not overbake). Cool on a wire rack.
In a small bowl, combine the first four frosting ingredients until smooth and creamy. Spread over bars. Sprinkle with walnuts. Store in the refrigerator.