Preheat oven to 350°F. Grease and flour 9X13 pan.
Sift together the sifted flour, sugar, brown sugar, cocoa and baking soda and set aside.
Heat the Dr Pepper in a small saucepan over low heat. Add the chocolate chips and stir until melted. Remove from heat.
With an electric mixer, beat the eggs, buttermilk, oil and vanilla for 2 minutes. At lowest speed, slowly add the Dr Pepper mixture and beat for 1 additional minute. Gradually add the dry ingredients, and beat at medium speed for 2 minutes.
Bake for 30 to 35 minutes, or until a toothpick comes out clean. Cool and allow to cool completely before frosting.