Take a chocolate digestive.
Add a dollop of buttercream (vanilla or chocolate). Use some Matchmakers broken up to represent the logs of the bonfire. The new(ish) honeycomb ones work well, from a flavour perspective, otherwise the mint can be overpowering. At this point you can dust with icing sugar to represent the ash.
Then using some torn up red shoelaces (the sweets, not actual shoelaces) place them around the chocolate logs to represent the flames.
Add some (fallen) mini marshmallows around the edge.