In a large bowl, cream butter and sugars until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine flour and baking powder; gradually add to the creamed mixture. Stir in zucchini, coconut and nuts. Spread into a greased 15x10x1-in. baking pan.
Bake at 350° for 25-30 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack. For frosting, combine confectioners' sugar and cinnamon in a bowl. Stir in butter, vanilla and enough milk to achieve spreading consistency. Frost bars before cutting.