Preheat oven to 350 F. Spray a 9 x 13 baking pan with cooking oil spray.
Melt butter in medium size bowl, add a cup of oatmeal. Carefully pour boiling water and stir until liquid is absorbed by oats. Allow oatmeal mixture to cool completely before proceeding, about 20 minutes.
Combine all-purpose flour, sugar, brown sugar, baking soda, spices and oatmeal-water-batter mixture in a large bowl. Stir well. Add in eggs combining completely.
Pour cake batter into prepared pan, spreading evenly. Bake for 25 to 30 minutes until cake springs back to the touch.
Combine evaporated milk, butter and sugar in a saucepan over medium heat. Allow butter to melt completely then add coconut, nuts and vanilla.
Stirring until the mixture becomes thick and sticky. Pour topping onto hot cake and spread evenly.
Place under broiler, carefully watching, not to burn until the topping is bubbly and evenly browned. Allow cake to cool completely before slicing.