Soak the Apples: Add the salt and vinegar to a very large bowl, Dutch oven, or plugged clean sink (anything large enough to submerge all the apples in water). Add enough hot water to dissolve the salt and cover the apples. Add the apples and let them soak for 20-30 minutes. Meanwhile:
Prep Stations: Line a baking sheet with a nonstick silicone baking mat. (You may use greased parchment paper if you don’t have one, but a baking mat is preferred because it sticks less). Prep all the toppings by dividing them into separate bowls and placing them next to the caramel-dipping area. Remove the Wax Coating: This step is crucial! Removing the wax coating from the apples entirely allows the caramel to stick instead of sliding off. Drain the apples, then use a kitchen cloth to vigorously rub the skin under running warm water to remove the wax coating.
Add Sticks: Wipe the apples dry, remove all stems, then insert apple sticks into the stems about ¾ through the apple core to create a handle. Refrigerate until ready to use. (Chilled apples help the caramel stick better. The apples can be stored for days at this point until ready to coat.) Prepare Caramel Station: Set a glass of water next to the stove with a silicone pastry or basting brush and set aside. Brush the sides of a 4-quart or larger heavy-duty saucepan with softened butter (helps to prevent crystallization). Combine Caramel Ingredients: Melt the brown sugar, sweetened condensed milk, dark corn syrup, butter, and salt in the pan over medium-high heat, stirring constantly with a wooden spoon. Cook Caramel: Once the butter has melted, reduce the heat to medium and attach a candy thermometer, ensuring it’s not touching the bottom of the pan. Continue to cook over medium heat, stirring often (make sure to get under the thermometer), until the candy thermometer reads 245°F. While cooking, occasionally brush down the sides of the pan with a water-moistened pastry brush. Add Vanilla and Cool: Once 245°F, remove from the heat and stir in the vanilla, taking care not to over-stir (to prevent bubbles). Allow the caramel to cool to 200–205°F. Meanwhile, wipe any condensation from the chilled apples.
Dip Apples: Working quickly, tip the pan as needed so that the caramel collects on one side. Holding an apple by the stick, dip it in the caramel and rotate 360° to coat all sides, then pull the apple straight up and swirl gently vertically to allow the caramel to drip off. Immediately transfer to the prepared baking sheet or coat in toppings:
Add Toppings: The caramel will set very quickly, so immediately roll the caramel-dipped apples in the toppings. I like to hold the apples over an empty bowl, sprinkle the top with the toppings because it sets the quickest, then roll them in the toppings. After the caramel has set, you can drizzle it with melted chocolate if desired.
Cool: Transfer the apples to the prepared baking sheet to cool and allow the caramel to firm up, about for 15 minutes; OR, I like to prop the apples up in a glass, stick side down (so the glass supports the stick, but the apple isn’t touching, see pics) to allow the caramel on the bottom of the apple to set and prevent the puddle effect.
Repeat and Reheat Caramel as Needed: Working quickly, repeat with the remaining apples. The caramel will thicken as it cools, so reheat periodically until it reaches about 190°F-200°F.