Make naleśniki batter: To a large bowl add the flour, sugar and salt and stir thoroughly using a whisk. Add the egg and oil then gradually and slowly pour in the milk whisking (quite vigorously) all the time until smooth.
Rest batter: The batter should be smooth and creamy and quite runny. Set aside for 15 minutes.
Puree cheese: Puree the twarόg in a food processor for a creamier consistency (this should take seconds).
Make filling: Transfer the cheese to a bowl, add the rest of the ingredients and stir thoroughly.
Fry crepes: Heat up about 2 teaspoons of oil spreading evenly in the pan then ladle the batter swirling it around to make sure it spreads over the entire surface of the pan.Don't be afraid to make these crepes quite thin, they won't tear easily. They need about 2 minutes on one side and a minute on the other (cooked on a medium heat).
Assemble: Spread a thin layer of the cheese filling over each crepe (2-3 tablespoons) and fold both sides over the filling (alternatively roll the crepes). Dust with a little powdered sugar (optional) and enjoy your naleśniki!
Notes
You can make these crepes large or small, depending on how big your pan is.
Once you've made the crepes batter set is aside for 15 minutes. This is a good time to make the filling.
Adjust the amounts of sugar and yogurt in the filling mixture according to taste. If using creamy substitutes for twarog (such as cottage cheese or quark) omit the yogurt.
Storing: Keep the naleśniki covered (it's best to wrap them in plastic to keep them from becoming dry), refrigerated for up to 3 days.
Make ahead: The filling as well as the naleśniki batter can be refrigerated, covered, for up to 3 days.
Reheating: either in the microwave or by frying in a little butter. You can also reheat the crepes as well as the filling separately then assemble and enjoy.
Serving: hot or cold.
Freezing: Freeze the filling for up to 3 months. I do not recommend freezing the batter. To freeze naleśniki place in an airtight container (separated with parchment paper so they aren't touching one another) and freeze for up to 3 months. Defrost in the fridge overnight. Reheat as above or serve cold.