Generously grease Bundt pan. Preheat oven to 350 degrees . In small bowl, stir cocoa into boiling water until smooth, then let cool completely. In large bowl, whisk barley flour, sugar, baking soda and salt. In center of bowl, make a well and add, in order, oil, egg yolks, cocoa mixture and vanilla. Beat with spoon until smooth.
Beat egg whites and cream of tartar until stiff. Fold a fourth of whites into batter to lighten it, then fold in rest. Pour into prepared pan. Bake on low rack for 60 to 65 minutes, until cake springs back when pressed and toothpick inserted into center comes out clean. Let cool for 20 minutes in pan before running a paring knife around edges and then inverting it onto cake plate.
When cake is completely cooled, stir together powdered sugar and cocoa in a bowl, then stir in vanilla and milk. Add milk just to make a thick glaze that can be poured over cake. Drizzle glaze over Bundt. Let dry for a few minutes before cutting.
