Step 1

Preheat the oven to 350-degrees F, and line a 8 or 9-inch round or square cake pan with a piece of parchment paper, trimming to fit inside the bottom of the pan. Brush a light layer of oil on the exposed sides of the pan.

Step 2

In a medium bowl, combine the butter and sugar. Beat with an electric mixer on HIGH until light and fluffy, about 2 minutes.

Step 3

Add the egg and vanilla, and beat just until combined.

Step 4

In a small bowl, stir together the Dr Pepper and milk. Set aside.

Step 5

In another small bowl, whisk together the flour, cocoa powder, baking soda and salt.

Step 6

Add one-third of the flour mixture to the butter mixture, and beat lightly to combine. Add half of the soda mixture, and continue beating. Add another third of the flour mixture, floured by the last of the soda mixture. Finally, end with the last third of the flour.

Step 7

Scrape the batter into the prepared cake pan, and bake on the center rack for 27-30 minutes, testing with a toothpick in the center before removing from the oven. The toothpick should come out mostly clean with no obviously wet batter. (Some fudgey crumbs are okay).

Step 8

Do not start making the frosting up the cake is done cooking. Once you pull the cake from the oven, add the butter, cocoa powder, and Dr Pepper to a small sauce pan. Turn the heat to low and melt everything together.

Step 9

Turn off the heat, stir in the powdered sugar until smooth and then pour on the warm cake.Let the frosting set until firm and then slice the cake and serve.