Preheat oven to 350 degrees (F). Generously grease a 9 x 13 cake pan; set aside.
In a stand mixer fitted with the paddle attachment, or in a large bowl using hand held beaters, cream the butter and sugars for 2-3 minutes, or until light and fluffy. Add the eggs and egg yolk; beat well.
In a small bowl whisk together flour, baking soda, salt and cocoa; add it to the wet mixture and stir until just incorporated.
Place Organic Oat Original Non-Dairy Beverage in a small saucepan over medium-high heat. Once rapidly boiling, add oatmeal, stir once, then remove from heat. Let the mixture stand for a few minutes, then add the oat mixture to the batter; mix well.
Stir in the espresso powder and half of the chocolate chunks.
Pour into prepared pan. Sprinkle the top of the cake with the rest of the bag of chocolate chunks.
Bake for 25-30 minutes or until a cake tester comes out clean and the surface springs back lightly when touched. Let the cake slightly cool while you're making the icing.
In a large saucepan over medium-low heat melt the butter completely. Whisk in the cocoa, stirring until completely combined. Remove the pan from heat and add the oat beverage, sugar, and vanilla. The icing will be thick, but keep stirring! If the consistency is just too thick and not pourable, add a splash more oat beverage. If it's too thin, add more sugar. Once it's at a pourable consistency, pour the icing over the cake, spread evenly, and add any sprinkles you may be using. This icing sets QUICKLY! So you'll have to move fast ;)
Let cake cool completely before cutting into slices and serving! Enjoy!
*to make dairy-free, use coconut oil or other vegan butter substitute.