To make the Chocolate Candy Cups:
Arrange 30 miniature candy cups (approximately 1 inch in diameter) on a baking sheet. For stability’s sake, I suggest using a double layer of cups for each candy (so you will need to buy a total of 60 cups to make 30 candies). This helps your chocolate cup to maintain its shape.
In a large nonreactive metal bowl, combine the chocolates. Set the bowl over a saucepan of simmering water and stir with a rubber spatula until the two chocolates have completely melted together and the mixture is smooth.
Remove the bowl from the simmering water and stir for about 15 seconds to release excess heat. Use either a small spoon or a pastry bag fitted with one of the smallest tips to fill the candy cups just under a quarter full with chocolate. Using a pastry brush, brush the chocolate from the bottom of each cup up the sides to completely cover the inside of the cup with chocolate. Place the cups in the refrigerator while you make the marshmallow filling. Set the remaining chocolate aside.
To prepare the Marshmallow Filling:
Attach a small plain tube tip to a clean pastry bag and set aside.
In the bowl of a standing mixer, sprinkle the gelatin over 1/3 cup cold water.
In a small saucepan over medium heat, gently stir together the sugar and ¼ cup water. Stop stirring and put a candy thermometer in the saucepan. Bring the mixture to a boil over medium-high heat and cook, still without stirring, until it reaches the soft ball stage, 235 degrees F.
Remove the pan from the heat and slowly stream it into the gelatin. Whisk vigorously for about 30 seconds to release excess heat, then place the bowl on the standing mixer fitted with the whisk attachment and mix on medium-high speed for 5 minutes. Add the vanilla and salt and continue to whisk for about 2 minutes longer. You do not want to whisk the marshmallow to soft peaks; it should be slightly looser than that. Working quickly, pour the marshmallow filling into the prepared pastry bag.
To assemble the Marshmallow Chocolate Candy Cups:
Pipe the marshmallow directly into the chocolate cups, filling each one a bit more than three-quarters of the way full. Gently knock the pan to level the filling.
If the reserved chocolate has hardened, set it over simmering water to remelt it. Spoon a top layer of the chocolate onto the marshmallow filling to cover it, gently knock the pan again, and place the cups back in the refrigerator to completely set.
The candy cups will keep, covered, in the refrigerator for up to 4 days. Generally speaking, they can be enjoyed directly from the refrigerator or after a few minutes at room temperature, but they will begin to melt or bloom if left unchilled for too long.