Wash and dry the apples thoroughly. To remove wax, dip briefly in hot water with a splash of vinegar, then dry completely. Let them air-dry 15 minutes. Insert sticks into the top center of each apple. Line a baking sheet with parchment paper or a silicone mat.
In a heavy-bottomed saucepan over medium-low heat, melt the butter. Stir in the brown sugar, corn syrup, sweetened condensed milk, and salt. Stir frequently with a sturdy spoon, scraping the bottom and sides.
Clip a candy thermometer to the pot and cook the caramel, stirring constantly, until it reaches 230–240°F (soft-ball stage).
Remove from heat and let the caramel cool until it reaches about 190°F. This is the best dipping temperature—hot enough to coat smoothly but cool enough to cling.
Tilt the pot slightly and dip each apple, rotating to coat evenly. Let excess drip back into the pan, then scrape the bottom lightly on the pot edge. Place apples on the prepared baking sheet.
While the caramel is tacky, roll apples in toppings like chopped nuts or cookie crumbs. For chocolate drizzle, wait 10–15 minutes before adding thin ribbons of melted chocolate.
Allow apples to cool completely at room temperature until glossy and firm. Package in clear boxes or wrap individually in wax paper for gifting or selling. Refrigerate for longer storage, up to 2 weeks. Let sit at room temperature before serving.