Thick cut, soft white or thick cut granary brown - either are delicious as long as they're thick slices and soft.
Enough Lurpack unsalted butter so that it melts into the bread along with the HP sauce - accept no substitute.
Unsmoked back bacon cooked WITH THE RIND LEFT ON so that the bacon is soft and the rind is crispy.
And THAT'S IT !
None of yer poncy egg or mushrooms, the question was the ultimate BACON sandwich not the ultimate Bacon and Egg or Bacon and Mushroom !!!
Actually - in the last couple of weeks I've just discovered an even better, brekkie sandwich.
Toasted brown bread, Lurpack as previous, Dijon Mustard (the sort with mustard grains in it) and a medium rare sirloin steak sliced up inside it - HEAVEN