Preheat the oven to 180°C and line a baking tray with greaseproof paper.
Melt the butter and chocolate in a bowl over a pan of simmering water without touching the base.
Whisk the sugar, eggs, and vanilla in a separate bowl and fold in the melted chocolate mixture.
Sift and fold the flour, cocoa, and baking powder into the wet ingredients.
Place tablespoonfuls of the mixture onto the tray to make 36 biscuits and bake for 6-8 minutes.
Place a marshmallow on half of the biscuits and return to the oven for 2-3 minutes until softened.
Press the remaining plain biscuits on top of the softened marshmallows and cool on a rack.
Drizzle with 100g of extra melted chocolate if desired and allow to cool completely.