The Kentish huffkin is named for its bread and the bread is the whole point, so it has to be understood as a roll first and a sandwich second. A huffkin is a soft, flattish, oval roll from Kent with a thumb-pressed dimple in the middle of its top, enriched a little so the crumb is tender and the crust stays soft rather than crisping hard. The dimple is not decoration: it is a shallow well, and in the traditional Kentish form it is filled, most famously with cherries from the county's orchards, so the roll is built around its own depression. The defining fact of this sandwich is that the container was shaped, deliberately, to hold the filling, which is the reverse of the usual order where a filling is fitted to whatever bread is on hand.
The craft is in the dough and the dimple. The enrichment keeps the roll soft for a day and stops the crust setting hard, because a huffkin is meant to yield to a tender filling rather than fight it, and a roll baked lean and crusty would not be a huffkin at all. The dimple is pressed before the final prove so it holds its shape through the oven and survives as a usable well rather than rising flat. Filled with cherries it is a sweet bread reading; split and filled savoury it works as a soft bap, the same tender crumb carrying ham or cheese. Either way the bread is doing the structural work and there is no need for butter to seal it, because nothing in the build is wet enough to threaten a crumb this soft.
The variations stay on the roll and change what sits in the well. The cherry huffkin is the orchard-county form, the dimple holding the fruit. A split-and-filled savoury huffkin treats it as a regional soft roll for ham or cheese. The wider family of soft enriched regional rolls is the broader context it sits in. Each of those deserves its own article rather than being crowded in here.