Cook the chips according to the package instructions (usually 25 minutes in the oven).
Spread the slices of bread with the butter, layer with the cooked chips, sprinkle with a little salt and top with another slice of bread.
If desired, top with a drizzle of ketchup, HP sauce or vinegar.
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Notes
For the most authentic chip butty, go for cheap, white sliced bread.Make sure the butter is room temperature, or it could make holes in the bread when spreading.Make sure the chips (french fries) are hot when assembling the butties.Try your chip butty with salt and vinegar, ketchup or HP sauce.