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1
Combine the olive oil, white wine, crushed garlic, rosemary sprigs and black pepper in a wide dish. Add the capretto joints and turn to coat. Leave to marinate at room temperature for 30 to 60 minutes — no longer. The marinade is done when the meat surface has taken on a faint sheen from the oil but the flesh remains firm and white.
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2
Cut four large sheets of heavy aluminium foil, each generously sized to enclose a portion with room for steam to gather. Season the sliced potatoes with salt, dress them with the additional olive oil and toss to coat. Arrange an even layer of potato slices at the centre of each sheet.
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3
Lift the kid joints from the marinade, allowing the excess to drain briefly. Lay the joints over the potatoes. Spoon some of the marinade over each portion. Tuck one fresh rosemary sprig alongside the meat. This is the step on which the dish lives or dies: fold the foil up and over the contents, crimping every edge firmly and repeatedly until the packet is completely airtight — no gap, no loose fold. Any escape of steam will dry the delicate milk-fed meat and prevent even cooking of both meat and potato.
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4
Place the sealed packets directly on an ember bed or set them on a baking tray in an oven preheated to 220 °C. Cook for 35 to 40 minutes. The packets should be visibly puffed and taut from internal steam pressure, a sign that the sealed environment is intact.
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5
Bring the packets to the table sealed. Open each cartoccio at the table: the aromatic steam — rosemary, wine and the milky sweetness of the kid — releases immediately. The meat should be opaque throughout with no trace of pink at the bone; the potato slices fully tender when pressed with a fork.