Preheat oven to 350∘F (175∘C). Grease and flour a 13x9 inch baking pan.
In a medium bowl, combine the oatmeal and boiling water. Stir well and let stand for 20 minutes. Do not drain.
In a large bowl, cream together the 1 cup brown sugar, 1 cup white sugar, and 21 cup shortening until light and fluffy.
Beat the eggs into the creamed mixture one at a time, beating well after each addition.
Stir the soaked oatmeal mixture into the wet ingredients.
In a separate bowl, whisk together the 121 cups flour, 21 tsp cinnamon, 21 tsp salt, and 1 tsp baking soda.
Add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.
Pour the batter into the prepared 13x9 inch pan and spread evenly.
Bake for 30 minutes.
While the cake is baking, prepare the topping: In a small bowl, combine the 6 Tbsp melted butter, 21 cup brown sugar, 41 cup milk or cream, 1 cup chopped nuts, 1 cup coconut, and 1 tsp vanilla extract. Stir well.
After 30 minutes, remove the cake from the oven. Immediately spread the topping mixture evenly over the hot cake.
Return the cake to the oven and bake for an additional 10 minutes, or until the topping is bubbly and lightly golden.
Let cool on a wire rack before cutting and serving.