1. Preheat oven to 180°C (160°C fan-forced). Lightly grease 2 oven trays and line with baking paper.
  2. For the chocolate biscuits, pour boiling water into a small saucepan, about a third full and sit a small bowl on top. Add Haigh’s dark pastilles and stir occasionally, until chocolate melts. Remove from heat and allow to cool slightly.
  3. Meanwhile, place cocoa, flour, sugar and butter together in a food processor and pulse until mixture resembles fine breadcrumbs. In a small bowl combine yolk, milk and vanilla, whisking until combined. Using a spatula, add to flour mix and pulse until combined. While the food processor is running, slowly add the melted chocolate until mixture starts coming together as a ball. Remove dough onto a lightly floured surface and bring together as a smooth ball. Flatten into a disk, cover with plastic wrap and refrigerate for 2 hours.
  4. Roll dough on lightly floured surface until 3mm thick, cut into 30 x 7cm rounds using a round cutter. Place rings on prepared oven trays and bake in preheated oven for 12 minutes. Remove from oven and place on a wire rack to cool. Leave oven on for marshmallows.
  5. To decorate, pour boiling water into a medium saucepan, about a third full and sit a medium mixing bowl on top. Add chocolate, stirring occasionally until chocolate melts.
  6. Spread 1 teaspoon jam onto the underside of 15 biscuits and set aside for later.
  7. Cut marshmallows in half horizontally. Place 4 marshmallow halves on the underside of the remaining biscuits. Place on a baking tray, marshmallow side up and return to oven for 2 minutes, allowing marshmallow to soften. Remove from oven and sandwich together with jam coated biscuits. Dip biscuit sandwiches into melted chocolate and place on baking tray lined with baking paper to set.