Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.
Step 2: In a large bowl, whisk together the flour, granulated sugar, 1/2 cup cocoa powder, baking soda, and salt. Set aside.
Step 3: In a medium saucepan, combine 1 cup butter, 1 cup Coca-Cola, and 1/4 cup unsweetened cocoa powder. Bring to a gentle boil over medium heat, stirring until the butter is melted and the cocoa is dissolved. Remove from heat and let cool slightly for 5-10 minutes.
Step 4: In a separate small bowl, whisk together the buttermilk, eggs, and 1 teaspoon vanilla extract.
Step 5: Slowly pour the cooled Coca-Cola mixture into the dry ingredients, mixing with a whisk or electric mixer on low speed until just combined. Do not overmix.
Step 6: Gently fold in the buttermilk-egg mixture until the batter is smooth. Pour the batter evenly into your prepared baking pan.
Step 7: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. While the cake is baking, prepare the frosting.
Step 8: For the frosting: In a saucepan, melt 1/2 cup butter, 1/4 cup cocoa powder, and 1/4 cup Coca-Cola. Bring to a boil, stirring constantly. Remove from heat.
Step 9: Gradually whisk in the sifted powdered sugar and 1 teaspoon vanilla extract until the frosting is smooth and creamy. If it’s too thick, add a tiny bit more Coca-Cola; if too thin, add more powdered sugar.
Step 10: As soon as the cake comes out of the oven, immediately pour the warm frosting evenly over the hot cake. Spread it gently to cover the top. The warm cake will absorb some of the frosting, making it extra moist.
Step 11: Let the cake cool completely before slicing and serving. This allows the frosting to set and the flavors to meld beautifully.