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1
Bake the potatoes in their skins until completely tender throughout, so the skin acts as a barrier and the flesh stays dry rather than absorbing water as it would if boiled. Halve them, scoop out the flesh, and pass it immediately through a potato ricer onto a clean work surface. Spread it and allow it to steam-dry for several minutes until dry to the touch — no surface moisture.
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2
Mound the riced potato and make a well at the centre. Add the flour, beaten eggs, and salt, then work the mixture together with your hands, drawing the flour in from the edges, so the dough comes together gradually and you can judge its texture before it is overworked. It should be firm and cohesive, not sticky; if it clings to the board, incorporate a little more flour in small additions.
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3
Stone each plum through the stem end using a skewer, keeping the plum whole so the skin holds its shape around the cavity. Press a sugar cube firmly into the space left by the stone, where it will melt during cooking and perfume the fruit from the inside.
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4
Divide the dough into twelve equal pieces and flatten each in the palm of the hand into a disc large enough to enclose one plum. Set the plum at the centre, bring the dough up and around it, and press firmly with both hands to close every seam completely — this is where the dish lives or dies, because any gap in the seal will allow water to enter during cooking, collapsing the plum and opening the dumpling. Roll each lightly on the board to neaten, then refrigerate the formed dumplings for 20 minutes so the cold firms the dough and sets the seams.
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5
While the dumplings rest, fry the breadcrumbs in 60 g of butter over a moderate heat, stirring constantly, until they are dark golden and crisp, so the butter toasts out of them and they hold their crunch even after being rolled around the hot, moist dumplings. They should colour deeply — pale breadcrumbs will stay soft and give no textural contrast. Tip onto a plate and reserve.
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6
Bring a large pot of lightly salted water to the boil, then reduce to a gentle simmer before lowering the chilled dumplings in in batches without crowding, so each one has room to move freely without knocking against its neighbours. Cook at a steady, quiet simmer for 10–12 minutes, keeping the heat low because vigorous boiling batters the seam open before the fruit has time to soften. The dumplings are ready when they have risen to the surface and floated there for a further two minutes.
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7
Remove the dumplings with a slotted spoon and drain briefly. Roll them immediately in the toasted breadcrumbs until fully coated, so the crumbs adhere while the surface is still moist and warm. Arrange on warmed plates, spoon over the melted butter, and dust generously with the mixed caster sugar and cinnamon. Serve at once.