Preheat the oven to 350 degrees.

Grease a 13x9 inch pan with vegetable shortening or butter. (This will keep the cake from sticking.)

In a medium saucepan, add the butter, Dr. Pepper, oil and cocoa powder.

Bring to a boil and then pour over the dry ingredients and mix well.

Add the baking soda into the buttermilk, stir and then pour into the batter.

Add in the eggs and vanilla extract and mix until the ingredients are well incorporated.

Pour into prepared pan and bake at 350 degrees for approximately 35-40 minutes.

Prepare your frosting while your cake bakes, since you'll want to add it to the warm cake.

Once the cake is baked, set on a wire rack to cool for about 15 minutes, then add frosting to the warm cake.

Add the four cups of confectioner's sugar to a large mixing bowl and set aside. In a saucepan, add the ¾ cup butter, 5 tablespoons of unsweetened cocoa powder and the ⅓ cup of Dr. Pepper. Bring the mixture to a boil and then pour it over the four cups of confectioner's sugar you added to your bowl. Add in the vanilla extract and mix well. Cover with plastic wrap until you're ready to add to to the cake. Once the cake is baked, cool it on a rack for about 15 minutes , then pour the warm frosting over the warm cake. Let the frosting firm up a bit before cutting and serving.