Beat butter at low speed with an electric mixer until creamy. Gradually add sugar; beat until blended. Add egg and vanilla; beat at low speed until blended.
Combine flour, cocoa, and soda. Add to butter mixture alternately with cola mixture; begin and end with flour mixture. Beat at low speed just until blended.
Stir in marshmallows. Pour batter into a greased and floured 13x9-inch pan. Bake at 350°F for 30 to 35 minutes. Remove from oven; cool 10 minutes. Pour glaze over warm cake; top with candied smoked pecans if desired.
Note: Don't make the glaze ahead--you need to pour it over the cake shortly after baking. Glaze – Mix 2 cups powdered sugar with 2 tablespoons softened butter, 1 teaspoon of vanilla, and 4 tablespoons of milk to a medium consistency.
Top with chopped smoked pecans. Pecans – Heat Pit Boss Grill to 250°F. Place pecans in a cast iron pan or aluminum tray, and mix to coat with 1 tablespoon of melted butter, 1 teaspoon of sugar, and one teaspoon of cinnamon metal or cast iron pan, and smoke for 45-50 minutes, shaking the pan every 15 minutes. Remove, let cool, and dice for a topping on the glaze.