Heat the oven to 325° F. Brush the inside of a Bundt pan thoroughly with oil or melted shortening and sprinkle with cocoa powder
In a medium bowl whisk together flour, sugar, brown sugar, cocoa, and baking soda and set aside.
In a medium saucepan heat Dr. Pepper and ½ cup chocolate chips over low heat. Stir until chips are melted. Remove from heat, and set aside.
In a large mixing bowl combine eggs, buttermilk, oil, and vanilla in a mixer bowl on medium speed until combined, about 2 minutes.
Add about half of the Dr. Pepper chocolate mixture and beat for one minute.
Add half of the dry ingredients and mix on medium speed until combined. Repeat with half of the Dr. Pepper mixture then the dry ingredients scraping down the sides of the bowl as needed.
Fold in the remaining 1 cup chocolate chips.
Pour batter into the prepared Bundt pan.
Bake at 325° for 45-50 minutes or until the cake starts to pull away from the center of the pan and a toothpick inserted into the center comes out clean.
Cool the cake in the pan for 10 minutes then lift and shake the pan from side to side to loosen it from the sides. If needed, gently run a knife around the edges to loosen cake.
Place cooling rack on top of pan and invert the cake on top. Remove pan and let cake cool completely.
Glaze
Combine powdered sugar and Dr. Pepper soda to make a glaze. Add more sugar if it is too thin, more soda if it is too thick.
Move cake to a serving plate and drizzle the glaze on top.
Notes
Substitutes for Buttermilk:
combine ½ cup milk and ½ cup plain Greek yogurt
add 4 tablespoons powdered buttermilk to the dry ingredients and use ½ cup milk and ½ cup Dr. Pepper into the liquid ingredients.