Spray a 9x13 baking pan with cooking spray. Set the oven to 350º Fahrenheit.
Pour the boiling water into a bowl or large measuring cup. Stir the oatmeal into the boiling water and set it aside to cool.
In a medium bowl, whisk together the flour, cocoa powder, baking soda and salt. Set aside.
In another bowl, cream together the butter, sugar, eggs and vanilla extract, scraping the bowl as needed.
Add the oatmeal to the creamed butter. Mix well.
Add the flour mixture to the butter/oatmeal mixture and mix again until combined.
Pour the batter into the prepared pan and spread it evenly.
Bake in preheated oven for 22-25 minutes or until the center of the cake springs back when you touch it.
Make the frosting:
*Plan to make the frosting about 15 minutes before the cake is done. The cake should be frosted while it is warm.
Melt the butter in a saucepan over medium low heat.
Add the cocoa powder and milk. Heat just until the mixture starts to bubble. Remove from the heat.
Using a hand mixer, gradually beat in the powdered sugar and vanilla until completely blended.
Spread the frosting on the cake as soon as it is out of the oven.
Allow the cake to cool completely.
Store any leftovers in an airtight container.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**