Wash the small Granny Smith apples thoroughly under cool water to remove any dirt. Dip them briefly in hot water mixed with vinegar for about 10 seconds, then dry each apple thoroughly.

In a heavy-bottomed saucepan, melt the unsalted butter over medium-low heat, stirring occasionally. Mix in brown sugar, corn syrup, sweetened condensed milk, and salt. Stir well until smooth and slightly bubbly (about 5 minutes).

Attach a candy thermometer to the side of the saucepan. Cook the caramel mixture while stirring constantly until it reaches the soft-ball stage, around 230-240°F, and shifts to a warm, golden amber color.

Remove from heat and let the mixture cool for a few minutes until it drops to about 190°F, ideal for dipping.

Gently tilt the pot of cooled caramel and dip each apple, rotating for an even coat. Allow excess caramel to drip off and place on a lined baking sheet.

Sprinkle toppings like crushed nuts or sprinkles while the caramel is still tacky. If drizzling chocolate, wait 10-15 minutes for the caramel to firm up slightly first.

Let the caramel apples cool completely until set and glossy (about 30 minutes). Enjoy immediately or store in an airtight container.