What is the name of this brown, "flat hat", fungus?
Pretty sure (but not 100% certain) that it is a Amanita porphyria bk9845, which in german is called a "Porphyrbrauner Wulstling" 


  Cap: 3-12 cm; convex, becoming broadly convex, often with a central bump; sticky at first or when wet; bald; appearing streaked radially; the margin not prominently lined; grayish brown to brown or purplish brown; often with scattered gray to purplish gray warts or patches.




Source: 


amanita porphyria