Preheat the oven to 350℉. Spray a 13x9-in. baking pan with nonstick spray.
In a large bowl, whisk together the flour, sugar, baking soda and salt. In a small saucepan, combine the Dr. Pepper, butter and cocoa powder; bring just to a boil, stirring occasionally. Add to the flour mixture, stirring just until moistened.
In a small bowl, whisk together the eggs, buttermilk and vanilla until blended; add to flour mixture, whisking constantly.
Scrape the batter into the prepared pan. Bake for 25 to 30 minutes or until a toothpick inserted into the center comes out clean. Set aside while you make the glaze.
ADD THE GLAZE:
In a small saucepan, bring the Dr. Pepper to a boil; cook 20 to 25 minutes or until the liquid is reduced to ½ cup. Stir in the butter and cocoa powder until the butter is melted; remove from heat. Add the powdered sugar; stir until smooth. Pour over the warm cake.