In the bowl of your stand mixer, combine gelatin, 1 cup cold water, coca, salt and vanilla. Stir well so there are no clumps of gelatin. Let it sit. Meanwhile, thickly coat the bottom of a 9x13 inch glass baking dish with 1 cup of sifted powdered . Attach a candy thermometer to a heavy bottomed sauce pan. Place the candy thermometer at least 1/2 inch off the bottom of the pan.
In saucepan combine sugar, corn syrup and 1/2 cup of water to a boil. Using a heat proof brush dipped in warm water, wash down the sides of pan to remove any sugar crystals. Cook sugar to 244 F. Remove from heat.
With mixer running on low speed, slowly pour syrup into mixing bowl. Slowly, increase speed to high and beat until mixture is very thick and white and has almost tripled in volume, about 15 minutes. Add vanilla; beat to incorporate.
Pour marshmallow mixture into pan and quickly smooth the top. Dust top with powdered sugar and let stand to cool and dry. Once cool and dry, turn out onto a cutting board. Cut marshmallows with a dry hot knife into squares, and dust with more confectioners' sugar.