The hundreds and thousands sandwich is sugar strands on buttered bread, and it is governed entirely by children's-party logic rather than by anything to do with lunch. Butter goes thick on soft white bread, hundreds and thousands are scattered across it until the surface is dense with colour, the bread is closed and cut into small squares or triangles, and the result is fairy bread: a thing made to look like confetti on a plate at a birthday and to be eaten in two bites by someone who is six. The defining component is the strands themselves, and specifically their snap. They are hard sugar, so against soft butter and soft bread they give the sandwich the one sharp textural event it has, a faint brittle crack under the teeth that is the entire sensory point of the build.
The craft is making the strands stick and keeping them crisp until they are eaten. Butter is structural here in a way it is in no other sweet sandwich: it is the adhesive, spread thick and right to the edges so the hundreds and thousands press into it and hold rather than rolling off a dry slice, and being salted it supplies the only counter to a filling that is otherwise pure sugar. The sandwich is assembled close to when it is wanted, because the strands draw moisture from the butter over time and the dye bleeds, so a fairy bread made too far ahead loses both its snap and its sharp colour and becomes a pale soft thing. The bread is the softest plain white available, because the appeal is partly visual and partly the contrast of crunch against something that offers no resistance at all.
There is not much to vary, which is honest to what the sandwich is. A different sweet topping over the buttered bread, sprinkles of another shape, or coloured sugar, is the same idea wearing a different speckle; the chocolate-spread version swaps the butter base for spread and loses the snap to gain a sweeter body. Each deserves its own article rather than being crowded in here.