1. Preheat the oven to 350 degrees Fahrenheit. Lightly grease a 17 1/2 by 12 1/2 inch jelly-roll pan.
2. In a medium sauce pan over medium-high heat, bring the cola, oil and unsweetened cocoa to a boil, stirring often. Remove from the heat.
3. In a large bowl, whisk together the flour, both sugars, baking soda and salt until blended; then add the warm soft drink mixture. Whisk in the buttermilk, eggs, and vanilla. Pour batter into the baking sheet.
4. Bake at 350°F for 18 to 22 minutes or until a wooden pick inserted in center comes out clean.

Make the Frosting

1. In a medium saucepan over medium-low heat, heat the butter and chocolate stirring constantly until melted and smooth. Remove from heat and whisk in the milk and the soft drink until blended. Stir in the sugar and vanilla. Beat at medium speed with an electric mixer until smooth.
2. Pour the frosting over the warm cake, spreading gently to the edges. Sprinkle with chopped pecans. Cool completely before serving.