Hi,
Pyroelectric differential sensors, as used in PIR detectors, can be used by covering one of the detectors (half the window) with a metal foil and then use the remaining as a magnitude sensor, but it takes a different circuit after the sensor and will need a bit of experimenting to get it right with whatever sensor you have - you may even have to adjust it for ambient temperatures right before using it.
Alternatively, get the right kind of sensor - some pyroelectric sensors are made for the exact purpose and they're quite efficient.
When I was around 6 years old, my father took me on a visit to a friends job. This guy was the Major on a large fire station (or what it's called) and demonstrated all of their new "toys" for my father and me, among them a pyroelectric based pocket size glow detector (for finding wood glowing inside without visible signs, used when the flames were gone and everything has to be positively quenched). This "toy" detected my fathers cigarette (lit, but not smoked, i.e. a minimal glow) 4 to 5 meter away -
that was around 1964, so it should not be hard detecting a much hotter candle flame today, with 46 years of improvements to electronics.