Preheat oven to 350. In small saucepan, melt butter . Stir in cocoa then water. Bring to a boil. Remove from heat. In large mixer bowl combine flour, brown sugar, baking soda, cinnamon and salt. Add cocoa mixture and beat well. Stir in 1/3 cup sweetened condensed milk, eggs and vanilla. Pour into greased 15 x 10 jellyroll pan. Bake 15 minutes or until cake springs back when lightly touched.
In small saucepan, melt remaining 1/4 cup butter. Stir in remaining 1/4 cup cocoa and sweentened condensed milk. Add 1 cup powdered sugar and nuts. Beat till smooth. Spread on warm cake.