01 - Sprinkle gelatin over 1/2 cup cold water in a stand mixer bowl and let it bloom for 10 minutes.
02 - Combine remaining 1/2 cup water, granulated sugar, corn syrup, and salt in a saucepan. Stir over medium heat until sugar dissolves, then boil without stirring until thermometer reads 240°F (115°C), about 8 to 10 minutes.
03 - With mixer on low, slowly pour the hot syrup into the bloomed gelatin. Add vanilla bean seeds or extract, increase speed to high, and beat until mixture is thick, glossy, and triples in volume, approximately 10 to 12 minutes.
04 - Lightly oil a 9x13-inch pan and dust it with confectioners sugar. Spread marshmallow mixture evenly, dust top with additional confectioners sugar, and let set uncovered at room temperature for 4 hours.
05 - Use a lightly oiled sharp knife to cut marshmallows into large squares and dust sides with confectioners sugar to prevent sticking.
06 - Whisk milk, cocoa powder, sugar, and salt in a saucepan. Heat over medium, stirring until steaming and smooth without boiling.
07 - Remove from heat and stir in vanilla extract.
08 - Pour hot chocolate into mugs, top each with a fluffy vanilla bean marshmallow floater, add whipped cream if desired, and serve immediately.