Turn your electric toaster up to its highest setting. Put the potato waffle in the toaster and turn the toaster on.
Meanwhile, spread your slices of bread generously on one side with butter.
When the waffle pops up at the end of the toaster's cycle, send it down again into the fiery chamber (still on the highest setting) for another full cycle or thereabouts. Towards the end of the cycle, be alert for the smell of burning in case the edges of the waffle begin to char.
When the waffle pops up for a second time, transfer it quickly to one of your buttered slices of bread (quickly, because the heat of the waffle must cause the butter to melt). Shake a little salt and vinegar over it.
Turn the waffle (but not the slice of bread) over and shake a little salt and vinegar over the other side. Top the waffle with the second slice of buttered bread (butter side down).
Tuck in. Marvel at the fact that your mouth thinks that it is eating a real, fried chip butty, while your head knows that your deep-fat fryer and oven languish, redundant, in the corner of your kitchen.