1 1/2 cups Dr Pepper
1 cup butter
1/2 cup unsweetened cocoa
2 cups all-purpose flour
2 cups granulated sugar
1 1/2 teaspoons baking soda
1 teaspoon ground cinnamon
1/2 teaspoon salt
2 large eggs, lightly beaten
1/2 cup whole buttermilk
2 teaspoons vanilla extract
Rich Fudge Icing
1/2 cup butter
1/4 cup unsweetened cocoa
1/4 cup Dr Pepper
2 tablespoons whole milk
4 cups powdered sugar, sifted
1 teaspoon vanilla extract
1 1/2 cups coarsely chopped toasted pecans
Prepare the Sheet Cake: Preheat oven to 350°F. Coat a 17 1/2- x 12 1/2-inch rimmed baking sheet with cooking spray.
Bring cola, butter, and unsweetened cocoa to a boil in a medium sauce-pan over medium-high, stirring often. Remove from heat.
Whisk together flour, sugar, baking soda, cinnamon, and salt in a large bowl.
Add cola mixture; whisk until blended.
Whisk in eggs, buttermilk, and vanilla.
Pour batter into prepared baking sheet.
Bake in preheated oven until a wooden pick inserted in center of cake comes out clean, 16 to 20 minutes. Remove from oven.
Prepare the Rich Fudge Icing:
Cook butter and cocoa in a medium saucepan over medium-low, whisking often, until butter melts and mixture is smooth, about 1 minute.
Reduce heat to low; whisk in cola and milk until blended.
Gradually add powdered sugar, whisking constantly until blended.
Whisk in vanilla.
Pour warm icing over warm cake; gently spread in an even layer.
Sprinkle with pecans.
Cool completely in baking sheet on a wire rack, about 1 hour.