• Preheat the oven to 350°F.
• Grease a 10x15-inch jelly roll pan.

Make The Sheet Cake

• In a large bowl, whisk together the granulated sugar, flour, baking soda, and salt.
• In a large saucepan, combine the Dr Pepper, butter, and cocoa powder.
• Bring the mixture to a gentle boil over medium heat, stirring until the butter has melted and the cocoa is completely smooth.
• Pour the hot cocoa mixture over the dry ingredients and whisk until combined.
• In a medium bowl, whisk together the buttermilk, eggs, and vanilla extract.
• Add the buttermilk mixture to the batter and stir just until smooth.
• Pour the batter into the prepared pan and spread evenly.
• Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Make The Chocolate Icing

• While the cake is baking, prepare the icing.
• In a medium saucepan, combine the butter, cocoa powder, and Dr Pepper.
• Bring the mixture to a gentle boil, stirring constantly until smooth.
• Remove from the heat and immediately whisk in the powdered sugar until smooth.
• Stir in the vanilla extract, then fold in the chopped pecans.
• As soon as the cake comes out of the oven, pour the warm icing evenly over the warm cake.
• Spread gently with an offset spatula if needed.
• Allow the icing to cool and set before slicing and serving.