In a bowl, cream butter and sugar. Add eggs, one at a time, beating well after each. Combine flour, cocoa and salt; add to creamed mixture. Stir in vanilla and nuts. Spread batter into two greased 13-in. x 9-in. baking pans. Bake at 325° for 25-30 minutes. Cool. In a bowl, cream butter. Combine sugar and cocoa; add to butter. Add milk and vanilla; beat until fluffy. Frost brownies.