Heat oven to 350 F. Grease and flour rectangular pan, 13x9x2-inches. In large mixer bowl combine flour, sugar, cocoa, baking soda and salt. Blend in butter, sour cream, eggs and vanilla. Beat 3 minutes on medium speed. Pour batter into prepared pan. Bake 40 to 45 minutes or until wooden pick inserted in center comes out clean. Cool in pan on wire rack. Frost with fudge frosting.
In small saucepan over low heat melt butter. Add cocoa; cook, stirring constantly, just until mixture begins to boil. Pour onto small mixer bowl and cool completely. Alternately add confectioners’ sugar, vanilla and milk, beating to spreading consistency.