Combine 1 cup of lukewarm milk and 1 packet of fresh yeast in a bowl; let it sit for about 5-10 minutes until bubbly and activated. In a large mixing bowl, whisk together 3 cups of flour, 1/4 cup of sugar, and 1 teaspoon of salt. Make a well in the center, then add the yeast mixture, 1 beaten egg, and 1/4 cup of melted butter. Mix until a shaggy dough forms.

Turn the dough out onto a lightly floured surface and knead for about 8-10 minutes until smooth and elastic. Place in a greased bowl, cover with a damp cloth, and let it rise in a warm environment for approximately 1 hour or until doubled in size.

Punch down the risen dough gently, roll out onto a lightly floured surface to about 1/2 inch thick. Cut out the donuts with a donut cutter. Place on parchment-lined trays, cover lightly, and let rest for about 30 minutes until puffy.

Heat oil in a fryer or heavy pot to 350°F (175°C). Fry the donuts in small batches, turning occasionally for 2-3 minutes per side until golden brown. Remove with a slotted spoon and drain on paper towels.

Once cooled to room temperature, fill the donuts with whipped cream using a pastry bag fitted with a filling tip. Dust with powdered sugar if desired.