To the bowl of your stand-up mixer (or a large bowl if you're using an electric hand mixer), add 1/2 cup of water and sprinkle the gelatin on top; allow to sit for 10 minutes. Attach the bowl to the stand-up mixer and add the whisk attachment.
Meanwhile, in a medium saucepan, combine the granulated sugar, corn syrup, salt and remaining 1/4 cup of water. Heat the mixture over medium heat until the sugar has dissolved. Turn the heat up to moderately high heat and bring the mixture to a hard boil and cook for 1 minute, until the candy thermometer or digital thermometer reads 240 degrees F.
Lower the whisk attachment and turn the machine on low. Carefully add the boiling liquid to the gelatin mixture. Turn the mixer to high and beat for 5 minutes, until the mixture has grown in volume. Stop the mixer and add 12 to 15 drops of each food coloring—this will make the perfect shad of “mint green."*If you're using a large bowl, simply add the hot mixture and turn the electric hand mixer on low being sure it doesn't splatter. After a minute or so, increase the speed to high and beat for 4-5 minutes. Proceed with recipe above and below.
Beat again for an additional 3 minutes, until doubled in volume and until it holds stiff peaks. Add the peppermint extract and vanilla extract. Beat for an additional minute or so until the both of them are incorporated. Fold in the cocoa nibs.
Pour the mixture into the prepared baking pan, smoothing out the top until it's evenly disbursed (it'll be sticky!). Sprinkle the remaining cocoa nibs on top. Lightly cover with plastic wrap.
Allow it to firm up on the kitchen counter for about 3 to 4 hours. Take a knife and run it around the edges of the casserole dish or pan and invert the marshmallow sheet onto a large cutting board, smacking the bottom of it, if needed. You also may need to use your fingers to loosen the marshmallow sheet from the casserole dish and gently glide it onto the cutting board.
Using a sharp knife, cut the marshmallows into 2 or 3-inch squares. Store the marshmallows in an airtight container for up to 1 week.