Preheat the oven to 350ºF. Grease a half sheet pan (18x13) liberally with butter or vegetable shortening. This cake does tend to stick more, so shortening is best.

In a medium saucepan over medium heat, heat the water, vegetable oil and cocoa powder until boiling. Stir to dissolve the cocoa powder. When it begins to boil, remove from the heat and allow it to cool while you prep the dry ingredients.

In a medium mixing bowl, combine the flour, granulated sugar, baking soda, and salt.

Gently pour the hot mixture into the dry ingredients, mixing with an electric mixer on low speed until combined.

Add in the sour cream, eggs and vanilla extract, mix again until combined.

Pour the batter into the pan and bake for 15-22 minutes or until a toothpick inserted in the center comes out clean. Make the frosting 5 minutes before the cake is done.

In a medium saucepan over low heat, heat the milk, cocoa powder, and butter until it just begins to boil. Remove it from the heat and stir in the powdered sugar, vanilla extract, and salt. Use a whisk to break up the powdered sugar clumps.

Pour the frosting over the warm cake. Allow the frosting to set completely for slicing and serving.