The nazis also went to Warsaw Ghetto and gave once a dinner to starving Jews and made a movie out of it to prove to their population that they were treated well and were complaining for nothing.
But if you want something specific, you can look on the site below, which starts with denials beginning 1942:
*1942–44 To conceal the evidence of their annihilation of Europe’s Jews, Germans and their collaborators destroy evidence of mass graves at the Belzec, Sobibor, and Treblinka killing centers, and at thousands of sites of mass shooting operations throughout German-occupied Poland, the German-occupied Soviet Union, and Serbia, including Babi Yar, in an operation code named Aktion 1005.
1943 In a speech to SS Generals at Poznan, Heinrich Himmler, Reich Leader (Reichsführer) of the SS (Schutzstaffel; Protection Squadrons), remarks that the mass murder of the European Jews will be kept secret, never to be recorded.*