Preheat the oven to 375F. Prepare a 10x15-inch jelly roll pan by spraying with non-stick cooking spray; set aside.

In a large bowl whisk together the flour, sugar, baking soda and salt.

Heat the butter and water in a medium saucepan over medium-high heat until the butter is melted and the mixture begins to simmer.

Remove from heat and pour into the dry ingredients mixture. Stir just until combined.

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

Pour the batter into the prepared jelly roll pan.

Bake the cake at 375F for 18-22 minutes, or until a toothpick inserted in the center comes out clean. Cool the cake in the pan for 5-10 minutes until warm.

Melt the butter and milk together in a medium saucepan over medium heat, then raise the temperature to medium-high until the mixture begins to simmer.

In a large bowl, combine the powdered sugar, vanilla extract, and the melted butter and milk mixture. Whisk together until smooth.

Pour the frosting over the warm cake and spread out evenly.

Cool the cake completely before slicing and serving.