Stir cocoa and boiling water until smooth; cool 20 minutes. Mix flour, sugar, baking soda, and salt in bowl. Make a well and add in order: oil, egg yolks, cocoa mixture, and vanilla. Beat with spoon until smooth. In a separate large bowl, beat egg whites and cream of tartar on high speed until stiff peaks form. Gradually pour egg yolk mixture over egg white mixture, gently folding with rubber spatula just until blended. Pour into ungreased 10- x 4-inch tube pan. Bake on lowest oven rack for 60–65 minutes or until top springs back when lightly touched. Immediately invert onto heatproof funnel or bottle until cake is completely cool. Remove from pan. For Chocolate Glaze, heat margarine in saucepan until melted; stir in powdered sugar, vanilla, and melted chocolate until blended. Stir in water, 1 Tbsp. at a time, until glaze is desired consistency. Spread over top of cake and let it run down over sides of the cake. Serve with light whipped topping, if desired.