Combine 1/2 cup warm water and instant espresso granules. Whisk until granules are dissolved. Transfer the mixture to the refrigerator to cool. Pour the cooled coffee into the bowl of a stand-up mixer (or you can use a large bowl and a hand-mixer). Sprinkle the gelatin atop the cooled coffee mixture and allow to sit for 10 minutes.
Meanwhile, in a medium saucepan, with a candy thermometer attached to its side, 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 reaches 240 degrees F.
Lower the whisk attachment and turn it on low. Carefully add the boiling liquid to the gelatin mixture. Turn the mixer to high and beat for 8-10 minutes, until the mixture has doubled in volume and holds stiff peaks. Sift in the cocoa powder and pour in the vanilla caviar or extract. Beat for an additional minute or so until the both of them are incorporated.
Pour the mixture into the prepared casserole dish or baking pan, smoothing out the top until it's evenly disbursed (it'll be sticky!). Dust the top with a few tablespoons of cocoa powder/sugar mixture. Cover with foil or plastic wrap and allow to set until firm, about 4 hours or overnight.
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 1-inch cubes. Store the marshmallows in an airtight container for up to 1 week.