Whisk in milk, a little at a time, and whisk in eggs until batter is smooth.
In a 7 inch non-stick pan, melt butter over moderately-high heat.
Half fill a ¼ cup measure with the batter and, off the heat, pour the batter into the pan.
Tilt and rotate pan quickly so that batter covers bottom in a thin layer and return any excess back into the bowl.
Return pan to heat, loosen edge of blintz with a spatula, and cook until underside is browned lightly.
Turn and brown other side.
Repeat, using up all of batter.
For filling: Combine farmers cheese, cream cheese and yolks until almost smooth and beat in sugar, salt, vanilla and lemon zest.
Spoon 2 tblsps of filling onto blintz; fold in 2 sides and roll up to enclose filling.
Put seam side down on a baking sheet.
Repeat, using up all of crepes and filling.
(blintzes can be made ahead and frozen at this point. Thaw and proceed with recipe.) In each of 2 large skillets, melt 2 tblsps of the butter over moderately-high heat, add blintzes and cook for 2 minutes.
Turn carefully and cook until golden and heated through.
Transfer to a heated platter and serve with sour cream and cherry preserves.