Preheat oven to 350 degrees. Grease 9×13-inch baking pan.
Combine the oatmeal and the boiling water and let the oats soak while you work on the rest of the recipe.
In a medium bowl, whisk the flour, baking soda, salt, ground cinnamon, and ground nutmeg.
Beat the butter, brown sugar, and granulated sugar in a large bowl using a handheld mixer on medium speed until blended. Add the eggs, maple syrup, and vanilla extract and stir to combine using a spoon.
Stir the softened oats into the mixture. Pour the mixture into the prepared baking dish. Bake for 35 minutes or until a toothpick inserted in the top comes out clean or with a few moist crumbs.
When the cake is almost done baking, stir the melted butter, brown sugar, milk, shredded coconut, and walnuts in a medium bowl.
Spoon the mixture over the warm baked cake, Use an offset spatula to level it. Place it under the broiler for 1-2 minutes. Keep a close eye on it as broilers are unpredicatable.
Notes
I only add maple syrup if I have some in the house. It is not a key ingredient, so do not purchase a bottle for 1 tablespoon.
After the cake has cooled, cover it with plastic wrap and store it on the counter for up to 3 days.
Freeze the cooled cake wrapped in several layers of plastic wrap for up to 3 months. Thaw on the counter overnight.
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