Make the Cake

1. Preheat oven to 350° F°. Coat a 9×13 baking dish with baking spray. Set aside.
2. Combine the Dr. Pepper, oil, butter, and cocoa powder in a large saucepan over medium heat. When it comes to a simmer, mix well and remove from the heat.
3. Whisk the sugar, flour, and baking soda together in a large bowl. Add the dry mixture to the saucepan and stir until combined with a rubber spatula or wooden spoon.
4. In a separate small bowl, lightly beat the eggs and buttermilk together. Add the buttermilk mixture to the saucepan and stir until combined and smooth.
5. Pour batter into the prepared pan and bake for 23-28 minutes or until a toothpick inserted into the center of the cake comes out clean.

Make the Chocolate Frosting

1. In a medium saucepan, combine butter, cocoa powder, and milk over medium heat, stirring occasionally until melted and smooth.
2. Whisk in the powdered sugar a little at a time, combining after each addition until smooth.
3. Pour over warm cake.
4. Serve warm or at room temperature.