To prepare cake layers:

1. Preheat oven to 350 degrees F.
2. Grease and flour two 9-inch round cake pans.
3. In a medium bowl, sift together flour, sugar, brown sugar, cocoa and baking soda.
4. In a saucepan over low heat, combine Dr Pepper and the chocolate chips.
5. Stir constantly until the chocolate chips are just melted.
6. Remove from heat and set aside.
7. In the bowl of a stand mixer fitted with a paddle attachment, combine eggs, buttermilk, oil, and vanilla extract on medium speed, about 2 minutes.
8. Reduce mixer to low speed, and slowly pour in chocolate mixture.
9. Beat until combined, about 2 minutes.
10. Gradually add the dry ingredients.
11. Increase mixer to medium speed, and beat for 2 additional minutes.
12. Divide the batter evenly between the two prepared pans.
13. Bake until a cake tester comes out clean, about 30-35 minutes.
14. Cool the cakes for 10 minutes before running a knife around the edges.
15. Flip the cakes onto a cooling rack and let the cake layers cool completely.

To prepare frosting:

1. Clean the mixer bowl.
2. In the mixer bowl of a stand mixer fitted with a paddle attachment, beat shortening and butter on medium speed until fluffy.
3. Add confectioners’ sugar and cocoa.
4. Continue mixing until combined.
5. In a small bowl, stir the Dr Pepper and vanilla extract until combined.
6. Slowly pour the Dr Pepper-vanilla mixture into the mixer bowl.
7. Increase the mixer speed to high to thin the frosting, about 1 minute.

To assemble:

1. Place the first cake layer on a flat plate.
2. Spread 1 cup of frosting on top.
3. Top with the second cake layer.
4. Spread remaining frosting evenly on the top and sides of the cake.