Heat oven to 375°F. Line 15-1/2x10-1/2x1-inch jelly-roll pan with foil; generously grease foil. Sprinkle linen or thin cotton towel with 1/4 cup powdered sugar.
Beat egg whites in large bowl until soft peaks form; gradually add 1/2 cup granulated sugar, beating until stiff peaks form.
Beat egg yolks and vanilla in medium bowl on medium speed of mixer 3 minutes. Gradually add remaining 1/3 cup granulated sugar; continue beating 2 additional minutes.
Stir together flour, cocoa, baking powder, baking soda, and salt;
add to egg yolk mixture alternately with water, beating on low speed just until batter is smooth.
Gradually fold chocolate mixture into beaten egg whites until well blended.
Spread batter evenly in prepared pan.
Bake 12 to 15 minutes or until top springs back when touched lightly in center. Immediately loosen cake from edges of pan; invert onto prepared towel. Carefully peel off foil.
Immediately roll cake and towel together starting from narrow end; place on a wire rack to cool completely.
for the marshmallow filling-
Stir butter, powdered sugar, and vanilla together to start incorporating.
Add in the marshmallow fluff and whip it good for 1 minute. Scrape sides and stir again briefly. If you think it's too thick, add in 1 tablespoon milk.
Unroll cooled cake, spread the frosting evenly across the entire thing, then roll up again gently. Wrap in plastic wrap and refrigerate 2-3 hours until the filling firms up. While cooling prepare the chocolate frosting.
for chocolate glaze/frosting-
Melt butter, chocolate chips, cocoa, and milk together in a small pot over low heat. Once everything has melted together, remove from heat then stir in powdered sugar and vanilla.
Once the swiss roll is cooled remove plastic wrap and place cake roll onto a cooling rack with foil underneath to catch any drips.
Pour warm frosting over cake, using the back of the spoon to spread gently, covering every exposed part of the cake.
Top with sprinkles if desired. Refrigerate 20-30 minutes until frosting has firmed up.