Before indulging again , get some more exercise with a 40-minute walk across the Vltava River to the Holesovice district for an apple tart ( 28 koruna ) at the Erhartova Cukrarna ( Milady Horakove 56 ; 420-233-312-148 ; www.erhartcafe.cz ) , a 1937-vintage confectionery that was renovated with pitch-perfect period décor in 2007 .