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.