Chocolate Marbled Angel Food Cake

• 14 egg whites
• 10 tube with removable bottom (pan)
• 1/2 batter an ungreased (light into)
• 350 gr sugar
• 170 gr batter
• 160 gr flour
• 45 ml boiling water
• 12 gr unsweetened cocoa powder
• 9 gr vanilla
• 5 ml cream of tartar