Line a 10x15-inch pan with foil. Preheat oven to 375°F. Melt butter and pour into foiled pan.
Mix nuts and coconut flakes and sprinkle evenly in pan. Drizzle with condensed milk.
Sift together flour, cocoa, salt, baking soda and sugar. Beat egg yolks in bowl until fluffy.
Blend sifted dry ingredients, water and vanilla and beat for another minute.
Beat egg whites separately until stiff and fold into mixture. Pour mixture into pan. Bake 20 minutes or until done.
Sprinkle with confectioner's sugar. Transfer to a cookie sheet, roll and wrap with towel to set.
Transfer to serving platter and sprinkle with more confectioner's sugar. Slice to serve.