Preheat oven to 350 degrees. Grease and flour a 9 x 13-inch baking pan.
CAKE: In a small bowl, pour boiling water over oats; let stand for 20 minutes.
In a medium bowl, whisk together flour, baking soda, baking powder, salt, cinnamon and nutmeg; set aside.
In a large bowl, beat butter, granulated sugar and 1 cup brown sugar until light and fluffy. Beat in eggs and vanilla. Add oatmeal mixture and flour mixture, beat until well combined, scraping the bowl occasionally. Spread batter evenly in prepared baking pan.
Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean.
Meanwhile, prepare topping: In a medium bowl, cream the butter and brown sugar until smooth. Stir in walnuts, coconut and vanilla.
Remove cake from the oven. Set the oven temperature to broil. Spoon topping over the hot cake; spread evenly to cover.
Broil 6-inches from heat for 1-2 minutes or until bubbly and light golden brown.