Add all ingredients to a large saucepan, and cook on medium heat until it reaches 220-230 degrees, or until the caramel is a "paste" type texture. **Please see note
Cut apples in half, and using a melon baller, scoop out the seeds, and some of the apple flesh (you want room to pour in the caramel). Baste the flesh of the apple with a bit of lemon juice to keep it from turning brown, but then dry the inside part where the caramel will go with a paper towel--this will keep the caramel from slipping off. Once caramel is ready, pour it into a large glass measuring cup with a spout, then pour caramel into each apple, filling to the brim. Place in fridge until cooled. Then cut into slices (if desired) and enjoy!!
It seems there is a lot of variability between thermometers. Instead of paying too close attention to the temperature, look closely at the consistency of the caramel. It needs to be thickened before you remove it from the heat! If it's not a thicker texture, it will not set correctly.