In a medium saucepan bring butter and water to a boil. Meanwhile, in a large bowl add flour and sugar and mix. Then add in eggs, sour cream, extract, baking soda and salt. Slowly pour in the boiling butter/water mixture. Stir with a spatula until well combined and smooth. No need to use an electric mixer. Batter will be very thin.
Pour into a greased 15-in. x 10-in. x 1-in. baking sheet. Bake at 375 for 18-22 minutes or until golden brown. Your cake will be flat and not very tall (it's a sheet cake). Cool for about 10 minutes. You still want your cake a little warm when you add the frosting so that it spreads well.
For the frosting: In a large saucepan, combine butter and milk. Bring to a boil. Remove from the heat, and then stir in the powdered sugar and extract. Stir until smooth and all combined. Spread immediately over warm cake. Decorate cake with sprinkles, if desired.