Preheat the oven to 350ºF (180ºC). Grease a 13″ x 18″ sheet pan with butter or spray oil.

In a small bowl, combine eggs, buttermilk, and vanilla until smooth.

Add eggs & buttermilk mixture to the cake mix and whisk until it's smooth.

Pour the batter into the prepared sheet cake pan. With a rubber spatula, gently smooth and spread the batter to the pan's edges.

Transfer to the oven and bake for 15-20 minutes or until a toothpick inserted into the center comes out clean.

Prepare the icing when the cake has approximately 6 minutes left to bake.

To a saucepan over medium-low heat, add butter and milk until butter is melted. Whisk in cocoa powder until combined, and immediately remove the saucepan from the heat. Continue to whisk in powdered sugar and vanilla extract until smooth. If adding pecans, stir in now.

Pour hot icing over the warm cake. Use an offset spatula or knife to quickly spread the icing over the entire surface of the cake before it sets. Allow frosting to set and harden (20-60 min), then slice and serve!