- Preheat your oven to 160°C fan / 180°C / 350°F. Line two large baking sheets with non-stick baking parchment.  
- In a large mixing bowl, cream together your softened butter and both sugars until light, fluffy and well combined. Add your egg and vanilla, then mix again until combined. Next, add your flour, cocoa powder and bicarb, mixing well to ensure there are no pockets of flour.  
- Break up your biscuits into small chunks by placing them into a bowl and bashing with the end of a rolling pin. Then mix them in alongside the chocolate chips, mini marshmallows and raisins, if using.  
- Divide your dough into 12 evenly sized pieces and roll each into a ball (mine weighed around 65g/2¼oz each). 
- Place the balls onto the prepared sheets, ensuring you leave adequate space between them to allow for spreading – I place three on each tray and cook in two batches. Bake for about 12 minutes. 
- Leave to cool on trays for 15–20 minutes before transferring to a wire rack to cool completely. 
- Once cool you can optionally dip the backs of each cookie in melted chocolate and allow to set on a cooling rack upside down. This can be done at room temperature or in the fridge.