Place the double cream and coffee in a small saucepan and bring to a gentle simmer. Take off the heat, allow to infuse for 15 minutes. Add the coffee cream back to the heat, then add the sugar and the butter to this. Bring to a simmer and let it bubble for about 5 minutes until thickened. Set aside to cool completely.
While the sauce is cooling, start your mousse. Put the marshmallows, butter, chocolate and water in a medium saucepan over low heat. Heat it up very gently until fully combined and melted together. Set aside in a large bowl to cool. Once it’s nearly cooled, whip the cream and fold through the chocolate mix. Refrigerate for at least an hour.
When you’re ready to serve, pop a scoop of mousse into each glass followed by an ice cream scoop, repeat once more. You’ll end up with two scoops of both per glass. Pour over the sauce, get those pecans on and add a little salt.