Fifteens is not a sandwich and there is no use pretending otherwise. It is a Northern Irish no-bake sweet, a chilled roll built from fifteen digestive biscuits, fifteen marshmallows, and fifteen glacé cherries, the count that gives the thing its name, bound with condensed milk and rolled in desiccated coconut. It earns a place in a catalogue of things eaten between or in bread only because it is sometimes treated that way in the home, and the honest thing to say up front is that its defining quality is the one a sandwich does not have: nothing in it is cooked. There is no oven and no heat anywhere in the method. It is set, not baked, and that fact governs everything about how it behaves.
The craft, such as it is, is the bind and the chill. Condensed milk is the entire structural system here, the role butter or gravy or mayonnaise plays elsewhere: it is the sticky, sweet glue that has to coat every crushed biscuit and every marshmallow piece evenly so the mixture holds as one body when it is rolled into a log. Too little and the roll crumbles the moment it is sliced; too much and it stays a slack, sweet paste that never sets to a clean cut. The biscuits are crushed but not to powder, because the point is a rubble texture against the soft give of marshmallow and the wet bite of cherry. The coconut on the outside is not decoration but a dry coat that stops the sticky roll adhering to everything and gives a faint resistance against the soft interior. It is wrapped, chilled until firm, and cut into rounds, the setting in the cold doing the work an oven would do for a traybake that was actually baked.
The variations stay inside the crushed, bound, chilled frame. A digestive swapped for a rich tea changes the rubble; a chocolate version works cocoa or melted chocolate into the bind; the quantities flex while the named ratio holds. Each tips the sweet toward a build with its own logic, and those deserve their own articles rather than being crowded in here.