Preheat oven to 350°. Grease a 15x10x1-in. baking pan. In a large bowl, whisk flour, sugar, baking soda and cinnamon. In a small saucepan, combine Dr Pepper, butter and cocoa; bring just to a boil, stirring occasionally. Add to flour mixture, stirring just until moistened. Add eggs, whisking constantly.
Transfer to prepared pan, spreading evenly. Bake until a toothpick inserted in center comes out clean, 18-22 minutes.
Meanwhile, for icing, combine butter, Dr Pepper and cocoa in a small saucepan; stir over medium heat until smooth. Transfer to a bowl. Beat in confectioners’ sugar, cinnamon and vanilla until smooth.
Remove cake from oven; place on a wire rack and let cool completely. Spread frosting evenly over cake; sprinkle with pecans.