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1
Boil the potatoes in their skins in well-salted water until completely tender. Drain, peel while still hot, and pass through a potato ricer onto a clean work surface. Spread and allow to cool until no steam rises — the surface of the mound should feel barely warm to the back of the hand.
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2
Make a well in the riced potato. Add the egg, flour, a pinch of cinnamon, and the lemon zest. Work together with a light hand, incorporating only as much flour as is needed to produce a smooth, cohesive dough that does not stick to the board. The dough should yield under a finger without springing back sharply.
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3
Stone each plum with care, keeping the fruit as intact as possible. Place one sugar cube in the cavity left by each stone. Select only plums that are ripe but firm — flesh that yields to light pressure but does not bruise or split at the stone opening.
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4
Divide the dough into 12 equal portions. On a lightly floured surface, flatten each portion into a round disc large enough to enclose one plum entirely. Lay a plum in the centre, gather the dough up around it like a parcel, and pinch every seam firmly closed. Roll gently between cupped palms to consolidate — the surface should be smooth and seamless, with no cracking or visible join. This is where the dish lives or dies: any open seam will allow the plum to cook out into the water.
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5
Bring a wide pan of lightly salted water to a gentle simmer. Lower the dumplings in batches, without crowding. Simmer — never boil hard — for 12 to 14 minutes. The dumplings are ready when they have risen to the surface and float with an even, rounded silhouette, and a skewer inserted to the centre meets no resistance at the dough layer.
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6
While the dumplings cook, melt the butter in a wide pan over medium heat. Add the breadcrumbs and a pinch of cinnamon. Stir continuously until the crumbs are deep golden and smell of toasted butter and warm spice. Remove from the heat immediately — the crumbs should colour evenly without darkening further.
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7
Lift the cooked dumplings with a slotted spoon. Roll each one in the pan of buttered breadcrumbs until entirely coated. Arrange on warmed plates, dust with caster sugar and a little additional cinnamon. Serve at once — when cut at table, the plum should release a burst of hot, sweet-tart juice into the breadcrumbed exterior.