Prepare a 9×13-inch glass baking dish with cooking spray then line with parchment paper and spray with more cooking spray. Leave some parchment paper overhang on the sides of the baking dishes so you can just pop the marshmallows out once set. Sprinkle the parchment paper with sifted confectioners sugar and set aside.
Put the sugar, corn syrup, salt and ¾ cup water in a medium saucepan with a candy thermometer. Bring the mixture to a boil over high heat, stirring with a wooden spoon to dissolve the sugar.
Cook without stirring until the mixture registers 238 degrees F on the candy thermometer, around 10 minutes.
While the sugar is cooking on the stove, place ¾ cup of water in the bowl of an electric mixer and sprinkle the gelatin on top. Leave it to soften for 5 minutes.
Take the saucepan off the stove and bring it over to the mixer. With the mixer on low speed, slowly pour the hot syrup into the gelatin mixture. Gradually raise the speed to high and beat the mixture until very stiff, around 15 minutes.
When the marshmallow is almost ready, melt the Toblerone chocolate in the microwave for 1 minute, stir until smooth and add the cocoa powder.
Beat in the vanilla extract then beat in the melted chocolate for 1 minute on high. Pour the mixture into the prepared glass dish.
Set the marshmallow aside, uncovered, for 3 hours.
Sift the remaining confectioners sugar onto your work surface and unmold the marshmallows right on the sugar.
Remove the parchment paper and with a sharp knife sprayed with cooking spray, cut 2-inch marshmallow squares.
Since the marshmallows are very sticky, you can roll them in the confectioners sugar to coat. The marshmallows will stay fresh in an airtight container for 1 week.