Begin by thoroughly washing and drying your apples. It's crucial they're completely dry so the caramel adheres properly. Remove the stems and insert wooden sticks firmly into the top of each apple.

Line a baking sheet with parchment paper and lightly grease it to prevent sticking. Set aside.

In a heavy-bottomed medium saucepan, combine sugar, corn syrup, butter, and sweetened condensed milk. Heat over medium heat, stirring constantly until butter melts and mixture is well combined.

Bring mixture to a boil, then reduce heat slightly and continue cooking while stirring frequently. Attach a candy thermometer to the side of the pan and cook until the mixture reaches 235-240°F (soft ball stage), about 20-25 minutes.

Once the caramel reaches the correct temperature, remove from heat and stir in vanilla extract and salt. Allow to cool for 2-3 minutes until slightly thickened but still pourable.

Dip each apple into the caramel, twirling to coat evenly. Allow excess caramel to drip off, then place on prepared baking sheet. Work quickly as the caramel will start to set.

If adding toppings, sprinkle them onto the caramel immediately after dipping each apple, before the caramel sets.

Allow caramel apples to cool completely at room temperature for about 1 hour, then transfer to the refrigerator to set fully for at least 1 hour more.