Start by gathering all your ingredients and tools needed for the cake.
Preheat your oven to 350°F (175°C) and prepare a 10x15-inch jelly roll pan by greasing and dusting it with flour.
In a large mixing bowl, whisk together the flour, granulated sugar, baking soda, and salt. Then, add in the sour cream and eggs, mixing until well incorporated.
In a saucepan over low heat, melt the butter. Add water and cocoa powder, stirring until the mixture comes to a boil, then take it off the heat.
Allow the cocoa mixture to cool slightly before folding it into the flour and egg mixture until everything is well blended. Pour the batter into the prepared pan.
Bake in the oven for about 20 minutes or until a toothpick inserted in the center comes out clean.
While the cake is baking, prepare the icing by combining milk, cocoa powder, and butter in a large saucepan over medium heat.
Bring the mixture to a boil, remove it from the heat, and stir in the powdered sugar and vanilla extract. If you like, mix in some chopped walnuts for added texture.
Once the cake is baked, spread the warm icing evenly over the top.
Slice, serve, and enjoy your delicious Southern Chocolate Sheet Cake!