Make the leavened dough. Combine 100g of flour, warm milk, sugar and yeast and set aside for 40 mins in warm place. Instead of 20g of fresh yeast, you can use 6g of dry yeast. Heat the dairy butter to room temperature.
When the leavened dough grows, add the eggs, remaining flour, melted butter, and a pinch of salt. The dough should at least double in size.
Rinse, chop the mushrooms. The champignons can be used instead of porcini. Blanch them for 3 mins, and then fry in vegetable oil until they have a golden crisp.
Cook and then mash the potatoes. Peel, finely dice and brown the onion. Chop the dill. Combine all the ingredients. Sprinkle with salt and pepper.
When the dough is ready, dust the work surface with flour and knead it well with hands. Shape a dough tube, and cut into pieces of the same size.
Roll out thin ring of 12 cm across diameter. Make four cuts on every ring. Put a lot of filling in the middle of it. Put two opposite edges together.
You should tightly put the edges together to save the shape of the bun. Then stretch a little another two opposite petals of the dough and shape such a flower with opened core.
Shape other raw knyshi. Put them on the baking tray lined with baking parchment. Let them sit for 20-30 mins. After brush them with whipped yolk.
Heat oven to 200-220C, bake the knyshi for 20-25 mins. They should cook through and get a golden crisp. Serve hot.