First, pour the Dr Pepper into a saucepan over medium heat and bring to a boil. Reduce to a simmer and cook until reduce by half. You will need 3/4 cup. (I just occasionally pour the Dr Pepper into a measuring cup and if it's not to 3/4 cup, put it back in the saucepan and keep cooking.) Once reduced, set aside (or in the refrigerator) to cool. (This can be done the day before and kept covered if you need to break the steps up into two days.)
Preheat oven to 350 degrees and line three 6-inch or 8-inch cake pans* () with cooking spray. Line with parchment paper and more cooking spray.
Whisk together flour, cocoa powder, baking soda, baking powder, and salt in a bowl. Set aside.
In a medium-sized bowl, combine sugar, oil, buttermilk, vanilla, and eggs. Mix with an electric mixer until combined. Add half of flour mixture and half of the Dr Pepper to the sugar mixture and mix until just combined. Repeat with second half of flour mixture and Dr Pepper; mix well.
Divide batter evenly among the cake pans and bake for 25 minutes, or until center is set and a toothpick inserted into the center of each cake layer comes out clean. Cool cake layers completely before frosting.