Melt the butter, and golden syrup or light corn syrup in a saucepan.
Once the butter melts, add it to the chopped chocolate and leave to sit a few minutes to melt the chocolate. Once the chocolate is soft enough, whisk the chocolate and butter mixture until smooth and glossy.
Scoop out about 1⁄4 of the melted mixture and spread it into the bottom of a 9-inch square silicone cake pan.
Put the cookies into a zip lock bag and then break them into chunks a rolling pin.
Fold the cookie pieces into the remaining melted chocolate mixture in the mixing bowl, and then add the marshmallows.
Add the mixture to the pan and spread it with a rubber scraper.
Top with chopped nuts (if you are using them)
Refrigerate for 2 - 4 hours (or until firm) to set and cut into squares to serve.