A custard sando fills soft bread with cooked custard cream, the same yellow kasutado that hides inside a cream pan, set between slices instead of stuffed into a bun. It sits on the sweet end of the Japanese sandwich shelf and is best understood against its two close relatives: where a cream sando uses loose whipped cream and a fruit sando hides fruit in that cream, this one is built on a cooked, eggy, vanilla-scented custard with real body. That body is the whole character. The custard is rich and spoonable, holding its shape rather than melting, sweet but anchored by egg yolk so it reads as comforting rather than airy. The bread is soft, pale, faintly sweet shokupan. Neither is much alone; pressed cold into one slab they make a tidy little dessert with more weight and depth than the whipped-cream version it is often mistaken for.
The craft is in the custard and the cut. A good kasutado is cooked properly: yolks, sugar, milk, and a starch thickener brought up slowly and stirred until it is glossy, smooth, and thick enough to stand on a spoon, then chilled hard so it sets into a sliceable cream rather than a sauce. Vanilla carries the flavor; a clean version has no raw-starch chalkiness and no scrambled grain from rushed heat. The bread is fine-crumbed shokupan, crusts trimmed, and the custard is spread thick and level to the very edges so the cut face is an even yellow band with no dry corners. Then it is chilled again before slicing so the layer holds a clean line. A good one cuts to a smooth uniform stripe and stays soft because it was kept cold. A sloppy one is thin and bald at the edges, the custard either weeping into the bread because it was undercooked or stiff and pasty because it was over-thickened.
The variations work the custard without abandoning it. A diplomat-style version folds whipped cream into the set custard for a lighter, mousse-like middle that splits the difference with the cream sando. Bakeries scent it with vanilla bean, brown it lightly for a creme brulee note, or tint and flavor it with matcha or coffee. Add fruit and it tips toward the cross-section and fruit sando world; lean it all the way to plain sweetened whipped cream and it becomes a cream sando outright. Each of those neighbors is its own thing and deserves its own article rather than being crowded in here.