Stir together 1 1/2 cups boiling water and oats; let stand 20 minutes.
Beat 1/2 cup butter, 1 cup sugar, and 1 cup brown sugar in a large bowl at medium speed with an electric mixer until creamy; add cinnamon and next 3 ingredients, beating well.
Stir in flour, baking soda, and salt just until moistened; beat in oatmeal. Pour batter into a lightly greased 13- x 9-inch pan.
Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean.
Combine remaining 1 1/2 cups brown sugar, coconut, and remaining ingredients in a bowl; spread over warm cake.
Broil 8 inches from heat 2 to 3 minutes or until golden.