Preheat oven to 350.
Add boiling water to oatmeal. Stir slightly and let stand for 20 minutes.
Mix brown sugar, sugar, and shortening.
Stir in eggs.
Add oatmeal mixture and stir.
Mix in remaining cake ingredients.
Pour into greased 13 x 9 inch pan.
Bake for 35-40 minutes at 350F.
In a small saucepan, melt butter, sugar, and evaporated milk.
Cook until bubbles and remove from heat.
Add coconut, walnuts, and vanilla and stir.
Pour immediately onto slightly cooled cake.