Heat the oven to 375°F. Cook macaroni in a large pot of water with the tablespoon salt until just tender, then drain and toss with the melted butter. Cool for 5 minutes.
Grease the 12 wells of a standard muffin pan with the softened butter and dust with breadcrumbs, shaking loose crumbs across the wells to coat their sides.
Whisk egg, milk, filling salt, pepper and mustard. Fold into the warm macaroni with the cheddar.
Divide evenly among the wells, pressing lightly with the back of a spoon so the noodles settle without being crushed. Bake for 16 to 20 minutes until the edges are golden and the centers reach 160°F.
Rest in the pan for 10 minutes, then run a thin knife around each edge and lift out carefully. Serve warm; each cup counts as one serving.