The grilled PB&J inverts the one thing the plain PB&J was designed to do. The lunch-bag version is engineered to stay cold and intact for hours, with peanut butter spread to both slices as a waterproof seal against the jelly. Put that same sandwich on a buttered griddle and the seal stops being a defense and becomes the payload: the peanut butter goes molten, the jelly loosens to a hot syrup, and the soft bread crisps into a thin shell around a center that now flows. It is not a warmed PB&J. It is a sweet melt that happens to start from the same two ingredients, and the heat is the recipe.
The craft is the same patience problem a grilled cheese poses, solved with a filling that behaves very differently. The outside of the bread is buttered, or spread thin with mayonnaise, edge to edge, and the sandwich is griddled low and slow so the peanut butter is fully liquid at the moment the crust reaches deep gold; rushed over high heat, the bread scorches while the center is still cold and stiff. The jelly is the structural risk in the other direction: too much and it boils thin and runs out the sides into the pan, so the ratio runs heavier on peanut butter than the cold build does, because the peanut butter has to stay thick enough to hold the loosened jelly inside the shell. The bread stays soft and slightly sweet, sturdy enough to crisp but not so dense it overwhelms the molten center it now protects. This is a flat-top and skillet sandwich, made to order and eaten hot, because once it cools the shell goes leathery and the whole point is gone.
The variations stay inside the hot, sweet frame. A version with banana that softens and caramelizes against the griddle, one with a slice of cheese that turns it savory, a build that swaps jelly for marshmallow or chocolate and pushes fully into dessert. The wider sweet-sandwich shelf, the cold PB&J, the Fluffernutter, the banana and bacon build, runs the same anchor-and-soft-carrier logic without the heat. Those deserve their own articles rather than being crowded in here.