Place 80ml cold water in a small bowl and sprinkle over the gelatine. Set aside and allow the gelatine to hydrate while you make the meringue.
Combine the sugar, corn syrup/golden syrup and 80ml water in a very clean saucepan. Place over low heat, stirring occasionally, until the sugar and syrup melt. When no sugar granules remain, brush down the sides of the pan to dislodge any sugar and increase heat to high. Bring to a boil and continue to cook until the mixture reaches 120C on a sugar thermometer.
Place the egg whites and salt in the bowl of a stand mixer and whisk on low speed until frothy. When the sugar syrup is almost at temperature, increase speed to medium and whisk to soft peaks. Being careful not to splash the whisk, pour the sugar syrup in a steady stream. Place the hydrated gelatine in the saucepan - the residual heat from the sugar syrup will melt the gelatine - and pour into the meringue.
Finally, add the Baileys and continue to whisk until cooled.
Set aside the marshmallow while you prepare the hot chocolate. As it stands it will thicken, allowing you to dollop a generous helping atop the hot chocolate.
Alternatively, pour the marshmallow into a 20x20cm baking tin lined with baking parchment and liberally dusted with cornstarch. Set aside at room temperature for at least 4 hours, or better still, overnight. The set marshmallow can be turned out and cut into squares using an oiled knife. Prepared this way, your marshmallows will keep for up to a week in an airtight container.
To make the hot chocolate, combine the milk and Baileys in a saucepan over medium heat and bring to just below a simmer, being careful not to boil.
In a separate bowl, place the chopped chocolate. Pour a little of the hot milk mixture over the chocolate and whisk to emulsify as it melts. Combine the melted chocolate mixture with the rest of the milk mixture, whisking to thoroughly combine.
Divide the hot chocolate between mugs or glasses and top with a generous dollop of marshmallow. Use a blowtorch to lightly toast the marshmallow and serve while piping hot!