Preheat the oven to 350 degrees F and grease a 9x13-inch cake pan.
In a large mixing bowl, mix together all cake ingredients until fully incorporated, then pour into the prepared baking sheet.
Bake cake for 35-40 minutes or until a toothpick comes out clean.
While the cake is baking, melt butter in a medium sized saucepan.
On medium heat,whisk in sugar, Dr Pepper, cocoa, and vanilla.
Stir constantly until brought to a boil. Reduce heat and boil for 1 minute.
Remove from heat and cool for 25 minutes.
Pour the warm frosting over the warm cake and allow it to sit for a minimum of 30 minutes before slicing and serving.
This cake can be stored in an airtight container for up to 3 days at room temperature.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**