Step 1

Preheat the oven to 350 degrees Fahrenheit. Afterward, grease and line with parchment paper on a 9×13 baking dish.

Step 2

Afterward, heat a saucepan in medium heat and stir in the butter, oil, cocoa powder, and Dr. Pepper. After that, allow to simmer, stir well until incorporated, then remove from heat.

Step 3

After that, add the Chocolate cake box mix into the saucepan and continuously stir using a rubber spatula.

Step 4

In another bowl, whisk in the buttermilk and egg until smooth and combined. Gradually add the buttermilk mixture to the saucepan and mix until well incorporated.

Step 5

After that, pour the mixture into the cake pan and bake in the oven for about 25 to 30 minutes, or you can insert a skewer in the center, and if it comes out clean, then it is done.

Step 6

Finally, place in the wire rack to cool down; make sure that the cake is icy before cutting or frosting.