Brie and bacon is the sandwich where fat answers fat instead of acid answering fat. Most bacon sandwiches set a sharp counter against the cure: brown sauce, red sauce, a vinegary pickle, something sour to cut the salt and the rendered grease. This one removes that counter entirely and replaces it with more richness. Brie is a soft, bloomy-rind cheese with a high fat content and a mild, mushroomy, faintly tangy flavour that goes slack and liquid under gentle heat, and laid against salt-cured bacon it does not cut the meat so much as absorb it, smoothing the salt and the cure into something rounder and heavier. The defining decision is that absence of a sharp note. The sandwich commits to being rich rather than balanced, and the cheese, not a condiment, is what makes it its own thing.
The craft is a melt-and-restraint problem. Brie has to be warmed just enough to slump and coat without weeping its fat into the crumb, so it goes onto bacon straight off the heat and lets residual warmth do the work rather than a long grill that would split the cheese and soak the bread. The bacon is cooked until the fat has rendered and the edge has firmed, which matters more here than in a plain butty: with no acid in the build, that crisp, salty edge is the only contrast holding a soft, fatty cheese in check, and without it the whole sandwich reads as one heavy note. The bread is a sturdier carrier than a soft bap, a ciabatta, a baguette, or a firm bloomer, because molten high-fat cheese on top of rendered bacon fat is a great deal of grease and a tender roll would go to paste under it. Where a sharp accent does appear it is the exception that proves the rule, an admission that the fat-on-fat build sometimes wants rescuing.
The variations push the same indulgent reading. Camembert deepens and intensifies the cheese; a fruit element introduced alongside, cranberry or a chutney, is the point at which the sandwich abandons the pure fat-on-fat idea and brings the acid back. The plainer bacon rolls it grew out of, the brown and red camps and the egg builds, are the working version of the same starting point. Each deserves its own article rather than being crowded in here.