Begin by peeling and cutting the starchy potatoes into even chunks. Place them in a pot of salted water and bring to a boil over medium heat. Cook the potatoes for 15-20 minutes, or until fork-tender. Once done, drain and let the potatoes cool slightly.

Using a potato ricer, pass the warm potatoes into a large mixing bowl, ensuring there are no lumps. Allow the riced potatoes to cool completely, then incorporate the flour, egg, and fine sea salt. Gently knead the mixture until just combined.

While the dough cools, halve the ripe plums, removing the stones. Place a sugar cube and a pinch of cinnamon inside each plum half, then gently press them back together.

Divide the potato dough into manageable pieces, dusting your work surface with flour to prevent sticking. Flatten each piece into a disc, place a prepared plum in the center, and fold the dough over it. Pinch the edges securely to seal the dumpling.

In a large pot, bring salted water to a gentle boil. Carefully add the dumplings and allow them to cook until they float to the surface, about 4-6 minutes. Let them simmer for an additional 4 minutes.

While the dumplings are cooking, whisk together the sour cream and powdered sugar in a bowl until smooth and creamy. In a separate skillet, melt butter over medium heat, then add fine breadcrumbs and sauté until golden.

Once cooked, gently drain the dumplings and transfer them to a warm serving dish. Drizzle melted butter over the top, then sprinkle with the sautéed breadcrumbs. Serve alongside the sweetened sour cream.