Preheat the oven to 350° and oil a 9″x13″ baking pan.

Place the oats in a large mixing bowl. Bring the water to a boil in the microwave or in a small pot on the stove, then pour it over the oats. Let it sit for 10 minutes.

Add 1/2 cup butter and the brown and white sugars. Stir them until the butter is completely melted.

Once the butter is melted, stir in the eggs and vanilla extract. Make sure that the mixture has cooled to lukewarm at this point so that it doesn’t cook the eggs when you add them.

In a smaller bowl, stir together the all-purpose flour, baking soda, salt, and cocoa powder. Add it to the wet ingredients, along with 1/2 cup of chocolate chips, and stir until well combined. The batter will be fairly thin. 

Pour the cake batter into the greased 9″x13″ baking dish. Sprinkle the remaining chocolate chips and the walnuts (if using) over the top. Bake the chocolate oatmeal cake at 350° for  35-40 minutes, or until the center of the cake bounces back when you gently tap it with your finger. You can also test it by inserting a toothpick into the center of the cake. If it comes out clean, or with only a few moist crumbs on it, the cake is finished baking.

Allow the cake to cool before cutting and serving. It can be eaten warm or at room temperature. Store the leftovers in an airtight container in a cool, dark place. But don’t refrigerate it, as that can dry out the cake.