Preheat oven to 350 degrees and spray a 9x13 baking dish with non-stick baking spray.
Mix together all the cake ingredients until fully combined. Pour into the prepared baking dish.
1 chocolate cake box mix, 3.4 ounces box chocolate instant pudding, 3 large eggs, 3/4 cup vegetable oil, 8 ounces Dr Pepper
Bake in the preheated oven for 35-40 minutes or until a toothpick comes out clean when inserted into the center.
Frosting
Melt butter in medium-sized pan over medium heat.
1/2 cup salted butter
Whisk in sugar, Dr Pepper, cocoa powder, and vanilla keeping the heat on medium.
2 1/2 cups powdered sugar, 1/3 cup Dr Pepper, 1/2 cup cocoa powder, 1 tsp vanilla
Stir constantly until brought to a boil. Reduce heat and boil for 1 minute. Then remove from the heat and allow the frosting to cool for 25 minutes. Set a timer and make sure you don't let it cool much longer so it's still pourable.
Pour the warm frosting over the baked cake and add chocolate sprinkles or nuts as garnish (if desired) then allow the frosting to cool for at least 30 minutes before slicing and serving.
Notes
Make sure to pour the frosting in the center of the cake and then smooth it out so that it doesn’t all just end up in the corners. If it does end up all just running to the corners, know that those will be the sweetest pieces in the cake.Change it up by switching out the flavor of the frosting. This would be really good with a fruity frosting, buttercream frosting, or even just a regular whipped frosting.Add any toppings right after adding the frosting so that they can set with the frosting. If you try to add toppings after you the frosting sets, the toppings may just end up falling off.Store in the refrigerator covered with plastic wrap or a lid for up to three days.