Preheat oven to 375 degrees.
Line jelly roll pan with aluminum foil; grease generously.
Mix flour, cocoa, baking powder, and salt; set aside.
Beat eggs in small bowl with electric mixer on high speed about 3 minutes or until very thick and lemon colored.
Pour eggs into large bowl.
Gradually beat in sugar.
Beat in water and vanilla on low speed.
Beat in flour mixture on low speed just until batter is smooth.
Pour into pan, spreading batter to corners.
Bake 12 - 15 minutes or until top springs back when touched lightly.
Loosen cake immediately from edges of pan; turn upside down onto towel generously sprinkled with powdered sugar.
Remove foil carefully; cool cake completely.

Marshmallow Frosting:
Mix egg whites, sugar, cream of tartar, corn syrup, and water in non-aluminum 3 qt saucepan.
Cook over low heat, beating with electric mixer on high speed until stiff peaks form, scraping bottom and side of saucepan occasionally; remove from heat.
Add marshmallows; beat until smooth.

Cut cake crosswise into 4 rectangles.
Layer rectangles, spreading each with about 1 c frosting.
Frost top with remaining frosting.

Chocolate Glaze:
Heat chocolate and margerine in 1 qt saucepan over low heat, stirring occasionally, until melted; remove from heat.
Stir in powdered sugar and water until smooth.
Stir in additional boiling water, 1/2 teaspoon at a time, until desired consistency.
Pour over cake, allowing some to drizzle down sides.