Take a plain melon pan, split it, and pack the inside with whipped cream or custard, and you have a different animal: the filled melon pan. The crackled cookie shell and the soft sweet body are still there, still doing their two-texture job, but now they are a container as much as a pastry. Where the plain roll is restrained and eaten on its own, this one is built around a cold creamy core, which shifts it from a bread-shelf item toward something closer to a cream sando in a bun's clothing. The scored melon-rind crust still signs the name; the filling is what the order is actually about.
The build adds a third element to a balance that was already delicate. The shell and crumb follow the standard form: a thin sweet cookie sheet draped over an enriched bread ball, scored and baked so the top fractures while the body stays soft and light. Once cool, the roll is sliced, usually most of the way through so it opens like a book or a clam, and filled. The filling is either a firm whipped dairy cream, lightly sweetened so it tastes of cream and not sugar, or a proper vanilla kasutaado cooked thick enough to hold a clean line and not weep into the crumb. The skill is in not drowning the bread: the crumb is already tender and the crust is the only crisp thing in the assembly, so an overfilled roll goes soggy and the shell loses its crack within minutes. Done well the eat is layered, a sandy top, a soft body, and a cool band of cream through the middle, each still distinct. Done poorly the cream pools, the underside turns damp, and the whole thing collapses into one sweet mush with no contrast left to justify the crust.
Because the filling is perishable and the texture window is short, the filled version lives in the chiller more than the open shelf, and it is best eaten cold and soon, while the cream is set and the crust has not yet gone limp from the moisture beside it. It eats richer and heavier than the plain roll, and the sweetness is doubled, so it reads more as a small dessert than as a snack bun.
The variations are mostly a question of what goes inside. Vanilla custard and plain whipped cream are the common pair; bakeries also run matcha cream, chocolate, anko, fruit cream, or a custard-and-cream hybrid that eats lighter. Some bake the shell extra crisp specifically to survive the wet filling longer; others tint the crust to match the cream. The plain unfilled roll, where the whole point is restraint and the crack of the bare shell, deserves its own article rather than being crowded in here.