Preheat oven to 180ºC (180C). In small saucepan, melt 1 cup butter; stir in quarter cup 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; beat well. Stir in one third cup of the condensed milk, eggs and vanilla. Pour into greased 15x10-inch (2L) jelly roll pan. Bake 15 minutes or until cake springs back when lightly touched. In small saucepan, melt remaining quarter cup butter; add remaining quarter cup cocoa and remaining condensed milk. Stir in icing sugar and nuts. Spread on warm cake. Cool completely.