Spray a loaf pan with nonstick cooking spray, then line it with parchment paper. Spray the parchment paper. Set aside.
Combine 1/4 cup water with the gelatin in the bottom of a stand mixer fitted with a whisk attachment. Let stand for about 10 minutes for the gelatin to bloom.
Add the remaining water, sugar, and corn syrup to a saucepan and boil until it reaches 240°F on an instant-read thermometer, about 8 minutes.
Turn the stand mixer on low and slowly drizzle in the hot syrup. When it's all added, turn the mixer to high and whip until the mixture is white, fluffy, and has stiff peaks.
Scrape the marshmallow mixture into the prepared pan and refrigerate for about 2 hours or until set.
Turn the marshmallow onto a board or work surface liberally dusted with icing sugar. Cut into 8-10 squares and liberally dust each with icing sugar to prevent sticking.
To make the hot chocolate, add the heavy cream and milk to a saucepan and heat over medium heat until almost simmering.
Add the chocolate, cocoa powder, and sugar, mixing until the chocolate is melted. Adjust sweetness, then add the Baileys Irish cream.
To assemble, pour 4-6 ounces of hot chocolate into a mug and top with a marshmallow. Torch the marshmallow with a kitchen torch to toast it.