If using frozen blackberries let them defrost in a saucepan for 30 minutes. Then add the icing sugar and cook, on a moderate heat, for around 5 minutes until blackberries are soft and squishy.
If using fresh blackberries, cook as above with icing sugar and a tablespoon of water.
Push the cooked blackberries and juices through a sieve to create a blackberry coulis, Scrape the thick pulp from the outside of the sieve into the coulis as you go. Discard any seeds.
Heat your choice of milk until scalding, meanwhile chop the chocolate and drop into the base of a blender, along with vanilla extract. Then pour the hot milk over the chocolate and allow it to sit for 1 minute.
Take 1 Tablespoon of the blackberry coulis for garnish and add the rest into the blender with all other ingredients. Blitz until the chocolate has melted and the drink is frothy.
Pour into mugs and top with whipped cream, marshmallows, a drizzle of the reserved blackberry coulis and blackberry then enjoy!