Which sernik variation is described as Vienna-style and crustless?

The crustless, Vienna-style sernik variation is called wiedeński.

The Polish word wiedeński means “Viennese,” and sernik wiedeński is distinguished by its lack of a pastry or crumb crust. The cheese filling is baked on its own, producing a straightforward cheesecake whose texture and flavor come from the seasoned curd mixture rather than a separate base.

Sernik is traditionally made with twaróg, Poland’s fresh quark-like white cheese. Its recipes commonly emphasize eggs and butter instead of the cream or sour cream associated with many American cheesecakes. Because twaróg is drier and more granular than ordinary cream cheese, it gives Polish cheesecake a characteristic, substantial texture.

The other names describe different forms. Sernik krakowski, or Kraków-style cheesecake, is associated with a lattice crust on top. Sernik królewski, or royal cheesecake, uses cocoa pastry on the top and bottom. All belong to the same Polish cheesecake family, but only wiedeński is identified by the crustless format.