Fairy bread is a sandwich whose entire function is to be a children's party food, and once that is understood every decision about it makes sense. It is soft white bread, buttered, scattered with hundreds and thousands, and cut into triangles. There is no second slice and no filling in any normal sense. The sprinkles are not a topping in the way jam or paste is a topping; they are a hard, brittle, brightly coloured texture event laid against soft bread and salted butter, and the pleasure is almost entirely the snap of sugar shot against a yielding base. It is open-faced, eaten in one or two bites, and built in seconds, because the occasion it serves does not allow for anything more elaborate.
The craft, modest as it is, is the butter and the press. The butter is structural before it is flavour: it is the only adhesive in the build, and the hundreds and thousands have to be scattered onto it while it is soft and then pressed in gently so they bed into the surface rather than rolling off the moment the bread is lifted or tilted by a small hand. Spread thin and the sprinkles will not hold; spread to the edges and the whole face stays covered so there is no bare, disappointing corner. The salt in the butter is what stops the sugar reading as flat, the same quiet trick that runs through every sweet sandwich on the British shelf. The bread is deliberately the softest, plainest white available, because any chew or crust would fight a topping that has no business being resisted and would make a one-bite party food into a chore.
The variations stay inside the soft, sweet, scattered frame. A different sprinkle changes the colour and the snap; a thin spread of jam or chocolate under the hundreds and thousands tips it toward a sweeter, stickier thing; the closed, two-slice version trades the open scatter for a packed one. Each tips the sandwich toward a named build with its own logic, and those deserve their own articles rather than being crowded in here.