Preheat the oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking dish.

1. Combine wet ingredients: In a saucepan, combine the Coca-Cola, butter, vegetable oil, and cocoa powder. Heat over medium heat, stirring frequently until the butter is melted and the mixture is smooth. Remove from heat and set aside to cool slightly.
2. Mix dry ingredients: In a large bowl, whisk together the flour, sugar, baking soda, and salt.
3. Combine wet and dry ingredients: Slowly pour the warm Coca-Cola mixture into the dry ingredients, stirring to combine. Once mixed, add the eggs, buttermilk, and vanilla extract. Stir until everything is well incorporated.
4. Bake the cake: Pour the batter into the prepared baking dish and spread it evenly. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan while you prepare the icing.
5. Prepare the icing: In a saucepan, combine the butter, cocoa powder, and Coca-Cola. Heat over medium heat, stirring occasionally until the butter is melted and the mixture is smooth.
6. Add powdered sugar: Remove from heat and whisk in the powdered sugar and vanilla extract. Stir until everything is smooth and well combined.
7. Frost the cake: While the cake is still warm, pour the icing over the top. Use a spatula to spread it evenly, allowing it to melt slightly into the cake for an extra gooey texture. If desired, sprinkle chopped pecans on top of the icing.
8. Cool and serve: Allow the cake to cool slightly before serving. The icing will firm up as the cake cools.