You don't need to write a custom stream buffer; the simplest and most straightforward way is to make a class you can send output to by giving it a templatized single argument operator<<():
template <typename T>
std::ostream& operator<<(const T& data_)
{
m_outstream << m_severity << "\t" << getTimeStamp() << "\t" << data_;
return m_outstream;
}
Where m_outstream is some type of std::ostream (std::ofstream, etc). m_severity and getTimeStamp() are examples of what you can insert (and you can create a bog standard two argument operator<<() to format and output the severity type to an ostream).
This then allows you to do things like:
myLogger << "failingFunction(" << argument <<
"): Could not do something." << std::endl;
and you will get output on m_outstream that looks like:
WARNING 2012-01-03 19:32 failingFunction("argument value"): Could not do something.
Beyond this, you will want a way to set the severity, which can be as simple as a method called on the logger class you added the templatized operator<<() to. If you want to get really fancy, you can write your own manipulator that acts much like setw() would on a std::ostream. See http://www.math.hkbu.edu.hk/parallel/pgi/doc/pgC++_lib/stdlibug/man_6665.htm for a primer on writing manipulators.