Preheat the oven to 350 degrees F. Grease a 15x10 inch rimmed pan. For a half batch version, use a 9-inch square pan. For a quarter batch, use a mini sheet pan.
In a saucepan, heat the butter until it melts. then stir in the Dr Pepper. Set aside.
Meanwhile, in a large mixing bowl, thoroughly whisk together the flour, sugar, cinnamon, salt, and cocoa powder..
In a glass measuring cup or another bowl, combine buttermilk, baking soda and vanilla. Add milk mixture to flour mixture, and once blended stir in the butter/Dr Pepper mixture.
Spread evenly in the pan and bake for 25 to 30 minutes. If using a smaller size pan and halving or quarter, check at 20 minutes.
For the frosting, heat the butter, Dr. Pepper and cocoa in a large (3 quart) saucepan until just until boiling.
Gradually add the sifted powdered sugar, beating with a wooden spoon until mixture is smooth.
Spread on warm cake and let cool and set.
