• Preheat oven to 350°F. Grease two 9-inch cake pans and line with circle of parchment. Grease again and dust with flour.
• Whisk together the flour, sugar, brown sugar, cocoa powder, baking soda and salt. Set aside.
• Heat the Dr Pepper in a small saucepan over medium. When it starts to bubble, reduce heat to low, add the chocolate chips and stir until melted. Turn off the heat and stir in the butter. Set aside to cool for about 10 minutes.
• With an electric mixer on low speed, begin beating the eggs on medium. Drizzle in the oil and continue beating on medium for one minute, then beat in the vanilla. Gradually add the buttermilk, stirring or beating on low. At lowest speed (or use a whisk if you prefer), slowly add the Dr Pepper mixture and beat for 1 additional minute.
• Gradually add the dry ingredients (flour mixture), and beat until smooth by hand or using medium to low speed of mixer.
• Divide the batter evenly between the prepared pans, and bake for 30 minutes or until a toothpick comes out clean. It may take up to 38 minutes, but check at 30.
• Let cool in pans for 15 minutes, then (carefully!) invert onto cooling racks. The cake is crumbly right out of the pan. lift off pans Allow to cool completely before frosting. Note: I recommend putting the slightly cooled layers in the freezer before frosting. Since they are so crumbly, partially freezing them makes them easier to handle.
• The frosting requires cooling time, so you may want to get started on that right away

Silky Chocolate Frosting

• For the frosting, off the heat, combine the sugar, flour and cocoa in a small saucepan. Gradually add milk and whisk until smooth.
• Put the mixture over medium heat and cook, whisking constantly, until mixture thickens and boils. Reduce heat to low, cook 2 minutes, whisking all the while. Remove saucepan from heat; cool completely. This cooling period should take 40 minutes to an hour. You may want to scrape it onto a plate and cover with a sheet of wax paper. This speeds up the cooling and prevents a skin from forming.
• In a large bowl with mixer at medium speed, beat softened butter until creamy. Gradually beat in cooled milk mixture, vanilla and melted chocolate. Note: If you do all the steps above and get a gooey mess, it's probably because something was too warm. I've done this a couple of times in the past and the fix is to beat in the chocolate as directed, but put bowl in refrigerator for 30-40 minutes to chill. The chilled mixture will whip up into a delicious, silky frosting.