Butter a large baking dish (about 2-3 litres in size). Sift the flour, ½ tsp salt, caster sugar, baking powder and 6 tbsp cocoa powder into a bowl. In a separate bowl, combine the milk, melted butter, eggs and vanilla. Mix the dry and wet ingredients together, then pour into the baking dish. This can be covered with cling film and chilled for up to 24 hrs.
Heat oven to 180C/160C fan/gas 4 and boil the kettle. Take the pudding out of the fridge and uncover, if necessary. Sprinkle the muscovado sugar and cocoa evenly over the pudding, then pour 700ml boiling water over the top.
Bake for 35-45 mins until the pudding is puffy and firm in the centre. Scatter over the marshmallows and return to the oven for 30-second intervals until they are just melting. Serve straight away - it will be hot - with cream or ice cream