Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Step 2: In a large saucepan, combine 1 cup butter, 1 cup Coca-Cola, and 1/4 cup unsweetened cocoa powder. Bring to a boil over medium heat, stirring occasionally, then remove from heat.
Step 3: In a large bowl, whisk together the flour, granulated sugar, baking soda, and salt.
Step 4: Pour the hot Coca-Cola mixture into the dry ingredients and mix well with a whisk or electric mixer on low speed until just combined. Be careful not to overmix.
Step 5: In a separate small bowl, whisk together buttermilk, eggs, and 1 teaspoon vanilla extract. Add this mixture to the chocolate batter and mix until smooth.
Step 6: Pour the batter evenly into the prepared baking pan.
Step 7: Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The cake should be moist and spring back slightly when touched.
Step 8: While the cake is cooling slightly in the pan, prepare the frosting.
Step 9: For the frosting, in a medium saucepan, combine 1/2 cup butter, 1/4 cup unsweetened cocoa powder, and 1/3 cup Coca-Cola. Bring to a boil over medium heat, stirring until the butter is melted and the mixture is smooth. Remove from heat.
Step 10: Gradually whisk in the sifted powdered sugar until smooth. Add 1 teaspoon vanilla extract and mix well.
Step 11: While the cake is still warm (but not piping hot, just warm to the touch), pour the warm frosting evenly over the top of the cake. Spread gently with a spatula if needed. The warmth of the cake helps the frosting create a lovely, slightly melted glaze.
Step 12: Allow the frosting to set completely before slicing and serving. This may take 30-60 minutes at room temperature. Enjoy your Decadent Coca Cola Chocolate Cake!