Preheat oven to 350 F. Grease a 9x9-inch cake pan. Set aside.
To make the cake, pour boiling water over oatmeal. Let it stand for 20 minutes.
With an electric mixer (either hand or stand), beat together butter and sugars. Mix until well-combined.
Add vanilla and eggs. Mix well.
Add oatmeal mixture, and stir to combine.
Stir together whole wheat flour, baking soda, salt, cinnamon, and nutmeg.
Add to batter in one addition and stir until just combined.
Pour into prepared pan.
Bake for 40-50 minutes, or until a toothpick inserted into center comes out clean. Remove cake and turn oven to broil.
Toward the end of the baking time, make the topping.
In a medium bowl, combine the melted butter, brown sugar, cream or canned milk, nuts, and coconut.
Spread the topping on the hot cake. Return to oven and broil until bubbly, about 3-4 minutes. Be sure to keep an eye on the topping because it can overbrown quickly.
Remove from oven, and serve warm.
Oats: Old-fashioned oats are sometimes called "rolled oats".
Butter: I use salted butter in this recipe. If you prefer to use unsalted butter, I recommend increasing the salt in the cake to 3/4 teaspoon and adding a couple pinches of salt to the topping.
Brown sugar: I use light brown sugar in this recipe, but you could use dark brown if you prefer.
Cream/canned milk: I use either evaporated milk or heavy whipping cream, depending on what I have on hand at the time. I don't recommend using sweetened condensed milk.
Pecans: You can use walnuts, if you prefer. If you want to omit the nuts, you can use additional coconut instead. You may find that you need to increase the amount of cream/milk by 1-2 tablespoons if using additional coconut.
Coconut: I use sweetened shredded coconut. You can use unsweetened if you prefer a less sweet topping.