This function hardcodes Game/Content/ and optionally includes Engine/Content to search for maps to cook. This means that you cannot select a map from a content plugin to cook. Instead the code should be iterating the available mount points for the project and building a list of maps for each mount point. Pseudo code:
for (PackageRoot in FLongPackagePathsSingleton::GetValidLongPackageRoots())
{ Search recursively through PackageRoot adding all maps }Have content plugins mounted in your project and try to select one map from their content directory in the cook by the book settings of the project launcher.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-48558 in the post.