Use a tripod because this shot will call for a low shutter speed, and turn the flash off, it will ruin the mood. Be sure to use the fastest lens that you have because you’ll be able to work with a larger aperture, letting more light in. A 50mm lens would be ideal at f/1.8 or f/1.4. Try a shutter speed of 1/30 of a second. The more candles on the cake, the easier it will be to property expose the shot, so if it’s a first birthday, you may have to try using an additional light source.