To prepare the apples, thoroughly rinse the cold apples, and wipe well to remove the wax coating. Remove stem. Using a caramel apple stick, carefully push the stick into the apple, about 3/4 of the way. Place apples on a parchment-lined baking sheet with a baking rack.: The first sensory cue is the cool firmness of the apples in your hand and the faint sheen from any wax you remove. Proper washing and drying lets the caramel adhere, and pushing the stick in most of the way gives a secure hold so the apple does not spin while dipping. You will feel resistance as the stick enters the core, then a reassuring firmness when it is seated correctly. A common mistake is forcing the stick in too aggressively so it punctures through the bottom, which weakens the apple. Take your time, and place the prepared apples on a rack so excess caramel can drip evenly.
To make the caramel, in a large heavy-bottomed saucepan over medium heat, combine the heavy cream, brown sugar, corn syrup, butter, and salt. Stir constantly with a wooden spoon until butter is melted.: As the mixture warms you will notice the brown sugar dissolving and the aroma shifting to warm, toasty notes. The heavy cream and melted butter will fuse into a glossy base, and the sound is a gentle, steady simmer rather than a vigorous bubble. Constant stirring helps incorporate the ingredients and prevents hot spots that could burn the sugar. If you stop stirring you risk localized scorching that makes the caramel grainy or bitter. Use a wooden spoon or heatproof spatula and keep the flame moderate for an even cook.
Using a pastry brush dipped in water, brush the sides of the pan and place a candy thermometer into the mixture, ensuring it does not touch the bottom of the pan. Without stirring, allow the mixture to cook until mixture reaches 235 degrees, about 20 to 25 minutes. Remove from heat and gently stir in vanilla. Do not over-stir. Allow mixture to cool and thicken, about 10 to 15 minutes.: The bubbling will transform visually from small, rapid beads to larger, slower bubbles as the temperature rises, and the aroma becomes deeper and more caramelized. Brushing down sugar crystals prevents recrystallization which would ruin the texture. A candy thermometer not touching the pan bottom gives an accurate reading; if it reads lower because of contact you may undercook. When the mixture hits 235 degrees you have reached the soft ball stage that yields a chewy but set caramel. After removing from heat the caramel will still be fluid but will thicken as it cools, and stirring in the vanilla extract after heat preserves its fragrance. One common error is stirring too vigorously at the finish which can introduce air and alter texture, so stir gently.
To dip the apples, working one apple at a time, hold the apple by the stick, dip the apple into the caramel mixture, and thoroughly coat using a twirling motion. If using toppings, roll the apple into the topping.: You will feel the warmth of the caramel against the chilled apple, and the glossy coating will cling as you twirl. A steady, confident dip and twist ensures even coverage and prevents drips from concentrating on one side. Rolling in toppings immediately after dipping means they adhere while the surface is tacky. A typical mistake is letting the caramel cool too much before dipping, which results in a thick, clumpy coat that will not spread smoothly. Work methodically and keep a bowl of warm caramel gently heated if needed.
Set apple on prepared baking sheet with rack, ensuring the apples do not touch. Cool slightly and serve immediately, or allow to sit until caramel hardens, about 45 minutes. Enjoy day of for best results.: Placing the dipped apple on a rack lets excess caramel drip away, and you will hear the faint settling sound as the coating relaxes. The visual cue to watch for is the caramel losing its glossy sheen as it cools and firming to the touch. Serving while slightly warm gives a pleasing chew, while waiting for full set provides a firmer bite. Avoid stacking or letting apples touch, because they may stick together. If you plan to store them, do so briefly and keep them upright to avoid deformation.