Preheat the oven to 350°F. Spritz a 13 x 9 inch baking pan with cooking spray. Set aside.
Mix together the oats and hot water. Set aside for 15 minutes to allow the oats to soften and cool.
Meanwhile, sift together all purpose flour, baking soda, ground cinnamon, salt and nutmeg.
Using an electric mixer, cream together butter, evaporated milk, brown sugar, granulated sugar and vanilla.
Add the eggs one at a time beating well after each addition.
Add the cooled oats and dry ingredients to the creamed butter. Continue to beat with a mixer just until fully combined. Stop and scrape the sides of the bowl as needed.
Spread into the prepared pan. Bake for 35-40 minutes or until a toothpick inserted into the center comes back with moist crumbs.
To make the Topping: Melt butter in a medium size microwave safe bowl.
Add the remaining topping ingredients mixing until fully combined.
Spread the topping evenly onto the cake immediately after it's removed from the oven. Increase the oven setting to broil moving the rack to the upper third of the oven.
Broil for 3-5 minutes until the topping has melted and is bubbly. Do not walk away the topping can burn easily if left unattended.
Cool to warm or serve at room temperature.