My experience with this (as a C and C++ developer who's worked with embedded systems before) is those kinds of job postings tend to require an engineering degre

My experience with this (as a C and C++ developer who's worked with embedded systems before) is those kinds of job postings tend to require an engineering degree, and most electrical engineers I've talked to want to do hardware not firmware (and didn't like their programming classes). You could try either relaxing the degree requirements, or reaching a wider audience -- I generally don't see firmware jobs advertised on HN and StackOverflow and those are basically the only two places I ever pay attention to job ads. Depending on your existing volume of applicants you might also ask them to attach a semi-trivial C assignment to their application. Yes, this is accessible to anyone who can google, but you might weed out the disengaged who don't care enough to read the instructions that way.