Melon pan is the baseline sweet roll of the Japanese bakery case, and the thing to understand about it before anything else is that the name describes a look, not a taste. A soft enriched bun is wrapped in a thin sheet of sweet cookie dough before baking, and that crust is then scored in a shallow crosshatch so the surface cracks into a webbed pattern that resembles the rind of a netted melon. The roll underneath is plain sweet bread. In the standard form there is no melon flavor at all. The whole identity of the thing lives in the contrast between two doughs and in that scored shell, which is why it reads as one of the defining shapes of the kashipan shelf rather than as a fruit pastry.
The craft is the management of two doughs that bake at different rates. The base is a tender enriched bread dough, light and faintly sweet, raised so the crumb is soft and pulls cleanly. Over it goes a separate cookie layer closer to a bisuketto paste, sugar and butter and flour worked smooth, rolled thin and draped over the proofed ball so it cloaks the top and shoulders but leaves the underside as plain bread. The surface is dusted with sugar and scored, and in the oven the cookie sheet sets and fractures along those score lines while the bread beneath rises and pushes the cracks open. Done well the result is a clear two-texture eat: a top that is crisp and sandy and sweet, shattering slightly at the first bite, over a body that is soft, light, and only mildly sweet, the two never collapsing into one mushy thing. Done poorly the crust is thick and claggy instead of crisp, the pattern is faint or smeared, the bread is dense and dry, and the whole roll tastes of flat sugar with no contrast to carry it. The right balance is a thin shell over a generous airy crumb, eaten the day it is baked while the top still cracks.
A plain melon pan is meant to be eaten on its own, warm if possible, often as a mid-morning or after-school thing rather than as part of a meal. It carries a coffee or milk well precisely because it is restrained, and its appeal is texture and gentle sweetness rather than any single loud note.
The variations move in two directions. Some bakeries do tint or flavor the crust, adding actual melon, matcha, chocolate, or coffee so the name finally tells the truth, or pressing sugar pearls into the shell for extra crackle. Others change the body: a chilled version eaten cold, an extra-crisp shell baked dark, regional rolls built larger or smaller. The branch that splits a melon pan open and packs whipped cream or custard inside changes it from a roll into a filled pastry, and that deserves its own article rather than being crowded in here.