Preheat the oven to 350°F (180°C), or 160°C for a fan oven. Spray a 9x13 inch (22x33 cm) baking dish with non-stick baking spray.

In a large bowl, combine the dry ingredients: flour, sugar, baking soda, baking powder, and salt until well combined. Set aside.

In a medium saucepan, over medium heat combine a can of Coca-Cola with cocoa powder, butter, and salt. Let the butter melt completely, then bring to a boil and remove from heat. Allow it to cool for a few minutes.

After the Coca-Cola mixture has cooled, pour it into the dry ingredients and whisk until combined. Add the eggs, sour cream, and vanilla extract. Mix well using a whisk until all ingredients are well incorporated. The batter will be relatively thin.

Pour the batter into the prepared baking dish and bake for 25-30 minutes or until a toothpick inserted into the center of the cake comes out clean.

While the cake is baking, prepare the frosting. In a separate saucepan, add the Coca-Cola. Simmer over medium heat until it is reduced to half.

Add the butter, salt, cocoa powder, and vanilla extract. Set over medium heat, whisking continuously until the butter is fully melted and the mixture is smooth. Remove from heat.

Gradually add the powdered sugar, whisking in between each addition until the sugar is fully dissolved.

Once the cake is done baking, allow it to cool for a few minutes, then pour the frosting over the warm cake. If the frosting has thickened, you can add a splash of milk and warm it up while whisking on the stovetop over medium heat to become spreadable.

Let the cake cool completely and allow the frosting to set before serving.