Preheat oven to 350. With an electric mixer, cream the butter and sugars. Add the eggs and mix well.
Add flour, soda, salt and cocoa and mix until just incorporated. The batter will be very thick.
Add water to oatmeal and let stand for a few minutes. Once oats are soft, add oats/water mixture to the batter and mix well. Stir in half of the bag of chocolate chips (6 ounces).
Pour into a greased or buttered rectangular 9×13 cake pan. Sprinkle the top of the cake with the rest of the bag of chocolate chips (6 ounces).
Bake at 350 for 30-40 minutes or until the surface springs back lightly when touched and your house smells amazing. Do not overbake. Let cool completely.
For the frosting, melt the butter, sugar, and milk in a large saucepan over medium-low heat. Bring to a boil and boil for 30 seconds.
Reduce heat to low and add chocolate chips and marshmallows. Stir or whisk until frosting is smooth. Pour immediately over the cooled cake. The frosting crystallizes almost immediately as it cools so it’s important to get it on the cake while it’s still piping hot. Allow the frosted cake to cool for 5 minutes before serving. Or… not.