Empty 1 envelope hot cocoa mix onto a saucer. Put some water into another saucer. Dip the rims of 4 martini glasses into the water and shake off any excess. Dip them in the hot cocoa mix. (Save the remainder for garnish.)
In a blender, combine the remaining the hot cocoa mix, milk, vodka, and ice and blend until smooth. Divide the mixture among the prepared glasses and garnish with the marshmallows and remaining cocoa mixture.