Preheat oven to 350°F. Generously grease a 9x13-inch cake pan; set aside.
In a stand mixer fitted with the paddle attachment, or in a large bowl using hand held beaters, cream the melted butter, white and brown sugars for 2 to 3 minutes, or until light and fluffy. Add the eggs and egg yolk; beat well.
In a small bowl whisk together flours, first amount of cocoa powder, the baking soda and salt; add it to the wet mixture and stir until just incorporated.
Place first amount of oat milk in a small saucepan over medium high heat. Once rapidly boiling, add oats, stir once, then remove from heat.
Let the oat 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 to 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 cut up butter completely. Whisk in the second amount of cocoa powder, stirring until completely combined. Remove the pan from heat and add the confectioners' sugar, second amount of oat milk, and the vanilla. The icing will be thick, but keep stirring! If the consistency is just too thick and not pourable, add a splash more oat milk. If it's too thin, add more confectioners' sugar.
Once the icing is at a pourable consistency, pour 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!
This nutritional information refers to each serving of the entire recipe and not just the products used as ingredients.