The effect of glowing candles from a birthday cake can turn out to be subtle if taken in a proper way. Here are some of the tips I find useful.
Tips 1: Use the candle as light source
As the candle is the focal point of the image, do not use any flash. Using flash will completely destroy the ambient light from the candles. Light up the subject in dark environment.
Tips 2: Use tripod
Use a tripod or support it steadily if you are using slow shutter speed to let in enough light.
Tips 3: Compensate Exposure
To avoid underexposure result of the candle flame as bright spot, use exposure compensation.
Tips 4: Change white balance
Rather than using auto WB, try experimenting it with other setting such as tungsten, fluorescent etc.
Tips 5: ISO setting
Increasing the ISO settings helps to compensate for low light environments. Keep ISO under 400 to get reasonably clean shots.