A Viennese pastry shop, dancing sweets, a little boy who overindulges and a revolution by the lower pastry orders. An almost unknown Richard Strauss score. Décor and costumes by the pop-surrealist artist Mark Ryden. And a great title: “‘Whipped Cream’! It’s really wonderful,” Alexei Ratmansky said of his full-length work for American Ballet Theater after a long day of rehearsal last week.