Strip and finely chop the leaves of the Flat-leaf parsley; set aside.
Tear a piece of Bread into chunks, put into a small bowl and pour over enough White wine vinegar to soak; let sit for 3–5 minutes, then squeeze out the excess vinegar and reserve the softened bread.
In a mortar (or a mixing bowl) mash together the Anchovy fillets, Capers and a crushed clove of Garlic into a coarse paste, seasoning lightly with a pinch of Fine salt as you work.
Add the squeezed bread and the Egg yolk to the anchovy-caper paste and mix thoroughly until combined.
Begin to emulsify by slowly whisking in the Extra-virgin olive oil a little at a time; continue until the mixture becomes glossy and slightly thickened. You can also pulse briefly in a food processor, stopping before the sauce becomes completely smooth — a bit of texture is traditional.
Stir in the chopped parsley and taste; adjust acidity with a touch more White wine vinegar or seasoning with Fine salt if needed.
Transfer to a serving bowl, drizzle with a little extra olive oil if desired, and serve alongside grilled meats, fish, boiled potatoes or as a lively condiment with bread.
