Preheat oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large bowl with an electric mixer or in the bowl of a stand mixer, combine boxed cake mix, oil, eggs, 1 cup of Dr. Pepper, and chocolate pudding.
Spread the cake batter evenly into the prepared baking dish. Bake 30-40 minutes, until a toothpick in the center comes out clean. A slight dip in the center of the cake is normal.
Start making the Dr Pepper chocolate frosting while the cake is baking. In a medium saucepan, boil cocoa powder, butter, and ⅓ cup Dr. Pepper. Remove from stove after it boils and stir in vanilla extract.
In a large mixing bowl or stand mixer bowl, add powdered sugar and give it a whisk to smooth out any lumps. Pour the saucepan mixture over the powdered sugar and mix until well blended. Add Dr. Pepper to thin it out, if it is too thick.
Spread the warm frosting over top of the entire cake. Allow a few minutes to set, then slice and enjoy!
Keyword easy chocolate cake with cake mix, easy dr pepper cake
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