Preheat the oven to 350°F. Spray a 9×13-inch baking dish with nonstick cooking spray and set aside.
Place the oats in a small bowl. Pour the boiling water over the oats and let sit while you prepare the cake, about 10 minutes.
In a separate small bowl, whisk together the flour, cinnamon, baking soda, and salt. Set aside.
Use a stand mixer or hand mixer to beat the butter and sugars together until creamy. Scrape down the sides with a spatula, as necessary.
Add the eggs and vanilla extract and mix until smooth. Add the oat mixture and mix until combined.
Add the dry ingredients and beat until just combined. Don’t over mix. Pour the batter into the prepared pan.
Bake for 30 to 40 minutes or until cake is set and a toothpick inserted into the center comes out clean. Remove the pan from the oven and set aside while you make the topping.
Preheat the oven to broil. In a medium saucepan, combine the butter, brown sugar, and milk. Heat over medium-high, stirring until melted and smooth. Remove from heat and stir in the vanilla extract and coconut.
Pour the coconut topping over the warm cake and evenly spread with a spatula.
Place the cake back in the oven, under the broiler, for 1 to 2 minutes or until the coconut is toasted. Don’t walk away, this won’t take long!
Let the cake cool for at least 15 minutes before cutting and serving. Enjoy!