Pommes Duchesse (Piped Potato Rosettes Baked with Egg Yolk)

Variations

• Pommes dauphine: fold about half as much choux paste into the same mixture, drop spoonfuls into hot oil and fry. The choux puffs as it fries and leaves the inside light — the same paste that eclairs and churros are made of.
• Croquettes: roll the mixture into cylinders, coat in flour, egg and breadcrumbs, and fry. The yolk that binds the inside is what keeps it from coming apart in the hot oil.
• As a border: piped round the rim of a dish with a stew or a shellfish filling in the middle, then put in the oven — the classic presentation, where the mixture works as a wall.
• Piped ahead: pipe them onto the paper and refrigerate or freeze them, then bake straight from cold. The oven does the setting, so the two steps can be separated. Give frozen ones about five minutes longer.
• Sweet potato: works the same way, but sweet potato holds more water and needs longer drying, and its sugar brings the colour on faster.

Tips

• The same material as mash, with the opposite aim. Mashed potatoes are meant to spread softly, so they are loosened with milk. This mixture has to stand with its ridges up for twenty minutes at 200°C, so there is no milk in it at all. The only liquid is the yolk, and the yolk is the thing that holds the shape.
• Starch is not the only way to stick potato together. The first is the one the mash page describes, starch: "Shear them and amylose leaks out of the ruptured cells into the liquid and tangles into long chains — that is the glue." Aligot wants that glue — "Aligot spends ten minutes making exactly that gluiness on purpose." — and pav bhaji makes the same body with a masher. Duchesse does not want it. The cells need to stay separate, or the ridges will not stand clean and the potato will not eat light. The mixture still has to hold together, and the second glue is the yolk.
• The yolk binds inside the oven. Eggs puts the yolk's setting point at about 68°C. When it is piped the mixture is still soft; once the inside of the rosette passes that line in the oven, the ridges are fixed where they stand. That is why an unbaked rosette squashes under a finger and a baked one can be picked up.
• The ricer is not optional here. The mash page says "A ricer or a food mill pushes the cells apart without shearing them", which makes glue hard to produce by accident. This dish adds a second reason: a lump caught in the star nozzle breaks the ridge.
• The piping bag presses the cells too. Forcing the mixture through a narrow nozzle is a mild shear. Once is fine, but scraping a bad rosette back into the bag works the same mixture twice, and it turns glossy and slack. Keep the rejects for croquettes instead.
• The drying time is the height of the ridge. Leave the potato longer than mash's moment on the heat. Water left in it turns to steam in the oven and slumps the ridges, and the shape goes before the yolk can set.
• The colour starts at the tips. The thin points dry before the body does, and only a dry surface browns (browning); the protein and fat in the brushed yolk deepen it. Blunt ridges give a flat, even colour, which is why a nozzle with fine, close teeth is worth using.
• Use floury potatoes. Waxy ones do not separate readily, tear as they go through the ricer, and go gluey before the ridges are even piped.