Step 1

Preheat the oven to 350 degrees Fahrenheit (175°C). Grease a 10x15-inch sheet pan with butter or nonstick spray and dust with flour.

Step 2

Melt butter in a saucepan over low heat. Mix in the water and cocoa powder and bring the mixture to a boil. Set aside to cool slightly.

Step 3

In a large bowl, whisk together flour, sugar, baking soda, and salt. Gently beat in the sour cream and eggs.

Step 4

Add the cooled cocoa butter to the flour and egg mixture. Mix until well combined.

Step 5

Pour the batter into the prepared pan. Bake for 20 minutes or until a toothpick inserted into the center comes out clean.

Step 6

Meanwhile, mix milk, cocoa powder, and butter in a large saucepan over medium heat. Bring the mixture to a boil and remove from heat.

Step 7

Beat in the powdered sugar and vanilla until smooth. Mix in the walnuts, if using.

Step 8

Spread the icing evenly over the warm cake. Slice, serve, and enjoy!