Line an 8x8 pan with parchment paper, and lightly dust the insides with the cornstarch so the marshmallows won't stick to the paper. Set aside.
Fit a stand mixer with the whisk attachment. Pour in 1/2 c warm water and the gelatin, stirring them together until the gelatin dissolves.
In a medium pot over medium heat, combine the honey, eggnog, vanilla, and salt. Stir together briefly, and let the mixture simmer. Clip your candy thermometer to the side of the pot, making sure the bottom is submerged in the liquid so it can accurately read the temperature.
Simmer the mixture until it reaches 240 F. You can turn up the heat to medium-high or high if you need to, but do this gradually so that the mixture does not pass 240 F. Immediately remove the pan from heat when this happens.
Turn the stand mixer to medium-high, and slowly pour the eggnog mixture into the stand mixer, being careful not to splash it. Beat the mixture for about 10 minutes, until soft peaks begin to form. The mixture will transform from a liquid to a soft, fluffy cloud of goodness.
Pour the soft marshmallow fluff into the prepared pan. Spread it to fit the pan using an offset knife or your fingers. I find that it helps to slightly wet your knife or fingers first. Smooth the top.
Allow the mixture to sit at room temperature for at least 4 hours.
Lightly dust the top of the marshmallow with cornstarch, and cut the marshmallows into squares, or use a cookie cutter to make fun shapes!
Store in an airtight container for up to 1 week. Do not store in the fridge, otherwise they will dry out.