01 - Sprinkle gelatin over 1/2 cup cold water in the mixing bowl and let it soak for 10 minutes.
02 - In a saucepan, combine remaining 1/2 cup water, granulated sugar, corn syrup, and salt. Heat over medium, stirring until sugar dissolves. Increase heat and boil without stirring until syrup reaches 240°F on a candy thermometer, about 8–10 minutes.
03 - With mixer on low, slowly pour hot syrup into bloomed gelatin. Add vanilla bean seeds or extract, then increase speed to high. Beat until mixture is thick, glossy, and tripled in volume, approximately 10–12 minutes.
04 - Lightly oil a 9x13-inch pan and dust with confectioners sugar. Spread marshmallow mixture evenly, dust top with more confectioners sugar. Let set uncovered at room temperature for 4 hours.
05 - Using a lightly oiled knife, cut into large squares and dust sides with confectioners sugar to prevent sticking.
06 - In a saucepan, whisk milk, cocoa powder, sugar, and salt. Heat over medium, stirring until steaming and smooth, but do not boil. Remove from heat and stir in vanilla extract.
07 - Pour hot chocolate into mugs, top each with a marshmallow floater, add whipped cream if desired, and serve immediately.