What kind of attorney specializes in writing Wills?

The US doesn't have the specialties you find in most places, so they don't have certifications in most cases. Typically a Contract Lawyer writes a contract and a Probate or Estate Lawyer would generate a will. All lawyers can technically do either of them, they just may not be as comfortable with it as others. In Great Britain and the British Commonwealth it would be a Solicitor that specializes in that area.