Combine the sugar, corn syrup, salt, fresh mint sprigs and ½ cup of water in a small saucepan over medium heat. Cook, stirring occasionally, until the sugar dissolves completely. Remove from the heat and let steep 1 hour.
Return the syrup to medium heat, and cook until the syrup reaches 240F. Remove the mint with a fork and discard.
While the syrup cooks, combine the gelatin and ½ cup of cold water in the bowl of an electric mixer. Let sit 3-5 minutes to bloom.
With the whisk attachment, run the electric mixer at low speed. While the mixer runs, slowly pour the warm syrup into the gelatin. Once the syrup has been added, continue mixing, increasing the mixing speed a little at a time, until increased to the highest speed. Whip on the highest speed until the mixture is very thick, about 10 minutes. Add the mint extract, vanilla extract, and a little food coloring at a time until the desired color is reached, mixing to combine. Remove the bowl from the mixture and fold in the chocolate with a spatula until just combined. (Too much stirring will smear the chocolate and muddy the color. A tiny bit of smearing will happen no matter what, but that’s okay!)
Line a non-metal 8 x 12 dish with parchment paper so that it covers the bottom and the sides of the dish with 1-2 inches extra hanging over the sides. Dust the parchment well with powdered sugar through a sieve. Pour the marshmallow into the dish, and top with more more chocolate (optional) and more sieved powdered sugar. Drape a piece of cling wrap loosely over the top, and let sit overnight to set.
Once set, turn the marshmallow out onto a clean cutting surface dusted with powdered sugar. Cut into squares, and coat each piece with more powdered sugar to keep them from sticking to each other. Enjoy right away or store in an airtight container at room temperature for up to a week.