Preheat your oven to 350°F (175°C).

Grease a 9x13 inch baking pan with butter or cooking spray.

In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.

In another bowl, mix the Dr Pepper, vegetable oil, eggs, and vanilla extract.

Pour the wet mixture into the dry ingredients and stir until just combined.

Transfer the batter into the prepared baking pan and smooth the top with a spatula.

Bake for 30 minutes or until a toothpick comes out clean.

While the cake is cooling, prepare the frosting by beating the butter, cocoa powder, powdered sugar, Dr Pepper, and vanilla until smooth.

Let the cake cool completely before frosting.