Process the sugar in a food processor until finely ground. Add the flour, hot chocolate mix, salt and baking powder, and pulse just to combine. Add the butter, and pulse until it is all incorporated into the flour and the mixture has a sandy texture. Add the eggs and vanilla, and pulse until the mixture comes together in big clumps.
Turn the dough out onto a clean work surface, and knead a few times to form a ball. Divide the dough in half, and roll each half into a log about 8 by 1 1/2 inches. Roll each log up in a piece of parchment, and tightly twist the ends. Freeze until firm, about 2 hours up to 1 month.
Position oven racks in the top and lower thirds of the oven, and preheat to 350 degrees F. Line 2 baking sheets with parchment paper.
Cut 1 log into 1/4-inch-thick slices (you should get about 24). Arrange 12 slices on each prepared baking sheet, and bake until the cookies lose their sheen and stay together when lifted with a metal spatula, about 12 minutes. Let cool on the baking sheet until firm enough to transfer to a rack to cool completely. Repeat with the remaining log of dough.
Arrange 24 of the cookies, bottom-side up, on a baking sheet. Put a marshmallow half in the center of each. Bake until the marshmallows puff up and are hot. Gently press 1 of the remaining 24 cookies on top of each marshmallow to form sandwiches. Let the marshmallow centers cool while the cookies are on the baking sheet.
Microwave the chocolate and honey in a microwave-safe bowl in 30-second increments, stirring in between, until completely smooth. Use the back of a spoon or a small offset spatula to spread some of the icing on top of each sandwich. Store for up to 2 days at room temperature in an airtight container.
For best results, don't use hot chocolate or cocoa mixes that contain powdered milk or other dairy products, which can change the structure and texture of the cookies.
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