• 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.

Frosting:

• 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.