Here's a classic recipe for White Texas Sheet Cake with Boiled White Frosting and added walnuts!
Preheat oven to 350 degrees F. Place the walnuts (from the frosting ingredients) on a baking sheet and bake for 5 to 7 minutes, until toasted and fragrant. Set aside and let cool.
Grease a 15x10x1 inch baking sheet* and preheat oven to 375 degrees F. (If using a dark colored pan, set to 350 degrees F.)
In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and baking soda. Set aside. In a small mixing bowl, beat the eggs, sour cream, and vanilla extract together. Set aside.
In a small pot, bring butter and water to a boil, remove and stir into the dry ingredients. (Don't wash the pan! You'll use it for the frosting.) Add in the egg mixture and stir until combined. Spread the batter into the prepared pan and bake for 20 to 22 minutes or until toothpick comes out clean. Set the cake aside to cool slightly while preparing the frosting.
In a medium saucepan, bring the butter and milk to a boil. Remove from heat and whisk in the extract and the powdered sugar 1 cup at a time until smooth. Stir in the toasted walnuts. Pour over the cake and spread evenly. Let cool, slice, and serve.
*If you use a (larger) 17x13x1 inch baking sheet, just begin to check if it is done starting at 18 minutes.