In a small bowl, whisk together the cornstarch,confectioner's sugar, and 1 teaspoon cocoa powder. Using a pastry brush, lightly butter the bottom and sides of a 13"x 9" baking pan. Dust the pan with the cornstarch mixture. Evenly cover the bottom of the pan with 2/3 cup of grated chocolate.
In a small bowl sprinkle the gelatin over 1/3 cup of water; make sure the gelatin granules are evenly moistened. Put the egg whites in a large bowl of mixer.
In a small heavy saucepan, stir together the granulated sugar, corn syrup, and 1/2 cup water. Bring the mixture to a boil over high heat, washing down the sides of the pan occasionally with a wet pastry brush. Insert a candy thermometer (be sure bulb is completely submerged) When syrup reaches 230 degrees, begin beating the egg whites at medium speed. When whites are foamy, add the salt and continue beating until the sugar syrup reaches 245 degrees.
Remove the sugar syrup from the heat and quickly whisk in the softened gelatin until completely dissolved. Beat the sugar syrup into the egg whites in a slow stream at high speed. Add the vanilla, and beat the whites until just cool, about 5 minutes. Using a large rubber spatula fold in 2 Tablespoons unsweetened cocoa powder.
Spread the meringue in the prepared pan and smooth the surface. Let stand uncovered in a cool dry place until set, at least 2 hours.
Sprinkle a work surface with the remaining grated chocolate. Run a knife around the edge of the pan to loosen the marshmallow. Turn out the marshmallow onto the chocolate. Quarter the rectangle. Cut each quarter into 12 even pieces. Roll each piece inthe chocolate until completely coated. Set the marshmallows on a rack to dry for 2-3 hours before serving. (Make ahead: the marshmallows will keep in an airtight container for up to 1 week)
Variation: Microwave semi sweet chocolate chips and 1 Teaspoon solid shortening until just melted. Drizzle over marshmallow squares. Place on platter lined with wax paper and refrigerate for 20 minutes until set.