In a small saucepan, combine cocoa powder, water, granulated sugar, and cooking oil. Stir over low heat for 1 to 2 minutes or until sugar dissolves. Remove from heat; cool.
Meanwhile, in a large bowl, sift together flour, granulated sugar, baking powder, and salt. Make a well in center of flour mixture. Add the 1/2 cup cooking oil, egg yolks, cold water, vanilla. Beat with an electric mixer on low speed until combined, then at high speed about 5 minutes or until satin smooth.
Thoroughly wash beaters. In another large bowl, beat egg whites and cream of tartar until stiff peaks form (tips stand straight). Pour egg-yolk batter in a thin stream over entire surface of beaten whites; fold in lightly by hand.
Transfer one-third of batter (about 3 cups) to a medium bowl. Gently fold in cocoa mixture.
Turn half of the light-colored batter into an ungreased 10-inch tube pan. Top with half of the dark batter. Repeat layers of light and dark batters. With a narrow spatula, gently swirl through batters to marble, leaving distinctive light and dark areas throughout.
Bake in a 325 degree F oven for 65 to 70 minutes or until cake springs back when lightly touched. Invert cake in pan; cool thoroughly. Loosen cake from side of pan; remove from pan.
Spoon Chocolate Glaze over top of cake, allowing excess glaze to drip down side. After the glaze sets, combine powdered sugar and a little milk to make an icing of drizzling consistency, if you like. Drizzle over cake. Makes 12 servings.
In a small heavy saucepan, heat cut-up semisweet chocolate and butter over low heat until just melted, stirring occasionally. Remove from heat. Stir in sifted powdered sugar and hot water, 1 teaspoon at a time, until glaze is smooth and of drizzling consistency.