Where can the IIS Express configuration / metabase file be found?
The configuration file is called applicationhost.config. It's stored here:
My Documents > IIS Express > config
usually, but not always, one of these paths will work
%userprofile%\documents\iisexpress\config\applicationhost.config
%userprofile%\my documents\iisexpress\config\applicationhost.config
Update for VS2019
If you're using Visual Studio 2019+ check this path:
$(solutionDir)\.vs\{projectName}\config\applicationhost.config
Update for VS2015 (credit: @Talon)
If you're using Visual Studio 2015-2017 check this path:
$(solutionDir)\.vs\config\applicationhost.config
In Visual Studio 2015+ you can also configure which applicationhost.config file is used by altering the <UseGlobalApplicationHostFile>true|false</UseGlobalApplicationHostFile>
setting in the project file (eg: MyProject.csproj). (source: MSDN forum)