Toss the plums in cane sugar and set aside for 20 minutes.
Mash the potatoes with a potato masher and place in a large bowl. Sift the potato starch, rice flour, and tapioca flour and add to the mashed potatoes. Stir, then add the egg.
Using your hands, knead the dough. It should be smooth and slightly sticky, but still easy to handle. If the dough is too runny or too sticky, add more potato starch.
Transfer the dough to a clean work surface and roll out to a thickness of ¼ inch. Dust with potato starch, if needed. Using a glass, cut out little circles (about 4 inches in diameter). Fill each circle with 1 teaspoon of plums, press the edges tightly together, and form into a ball with your hands.
In a large pot, bring 5 cups of water to a boil over medium-high heat. Carefully add 4-5 dumplings at once. Cook, uncovered, for 7-8 minutes. Remove the dumplings with a strainer and drain. Continue until all the dumplings are cooked. You should get 12-15 dumplings.
Just before serving, heat a large pan over low heat. Add butter and cook for 1 minute until melted. Add almond flour and all the dumplings. Fry for 3-5 minutes, tossing a few times, until golden.
Transfer from the pan to 4 serving plates. Garnish with blueberries, drizzle with heavy cream and top with mint leaves.