Preheat oven to 350 degrees F. Grease and flour a 10x15 inch pan.
Combine the flour, sugar, baking soda and salt. Beat in the sour cream and eggs. Set aside. Melt the butter on low in a saucepan and add the water. Bring mixture to a boil then remove from heat. Allow to cool slightly, then stir butter mixture into the egg mixture and mix until well blended.
Pour the batter into prepared pan. Bake in the preheated oven for 20-22 minutes, or until a toothpick inserted into the center comes out clean. Cool in the pan on a wire rack. While the cake is cooling, prepare the frosting.
For the frosting: In a large saucepan, combine the milk and 1/2 cup butter. Bring to a boil, then remove from heat. Stir in the confectioners' sugar and vanilla, then fold in the nuts, mixing until blended. Spread frosting over the top of the cake.