Preheat oven to 350ºF. Sift 1 1/2 cups confectioners sugar, flour, cocoa, and 2 teaspoons cinnamon 3 times into a medium bowl. In a large bowl, beat egg whites, cream of tartar, and vanilla with an electric mixer until soft peaks form. Gradually add granulated sugar, 2 tablespoons at a time, and beat until stiff peaks form. Sift about 1/4 of confectioners sugar mixture over egg white mixture; fold in gently by hand. Continue to sift and fold in confectioners sugar mixture in small batches. Lightly spoon batter into an ungreased 10-inch tube pan and place on lower rack of oven. Bake 40 to 45 minutes, or until top springs back when lightly touched.  Remove from oven and invert pan onto neck of a bottle; cool completely. Remove cake from pan, placing bottom side up on a serving plate.
To decorate, combine remaining 1/4 cup confectioners sugar and 1/4 teaspoon cinnamon in a small bowl. Place a 10-inch round paper doily on top of cake and lightly sift confectioners sugar mixture over doily. Carefully remove doily. Garnish with sugared grapes. Serves 10-12.