Place the farmer's cheese, powdered sugar, cornstarch, and egg yolk in a medium bowl.
Mix until everything is combined, and you're done!
Notes
Makes about 1 and 1/2 pounds of filling.
Let the sweet cheese filling stand for a while on the kitchen counter at room temperature. Do not use it cold from the fridge; the dough under the filling might not bake evenly, especially if it is yeast dough.
The resulting cheese filling should have a creamy but still firm consistency. Any small amount of extra moisture does not matter; the starch and egg yolk will stiffen it during baking.
STORAGE: The sweet cheese mixture can be made ahead of time. Then store it in an airtight container in the fridge and use it within three days. Let the filling come to room temperature before spreading it on the dough.