Stick a lollipop stick into the center of each apple and place them on a parchment-lined baking sheet. Return to the refrigerator while you prepare the caramel.
Affix a candy thermometer to the side of a large, heavy saucepan.
Stir together the sugar, water, and salt in the saucepan. Place over medium heat and whisk constantly until the sugar has dissolved and the mixture comes to a rolling boil, then stop whisking.
Allow the mixture to boil until it turns a light golden brown, swirling the pan occasionally to assess the color, then slowly and carefully add the heavy cream (the mixture will bubble up ferociously).
Reduce the heat to medium-low and cook, stirring constantly with a rubber or silicone spatula, until the mixture reaches 246 degrees F. Remove from the heat and stir in the vanilla extract.
Dip the apples in the caramel and return to the parchment-lined baking sheet, or roll in desired toppings. Allow the caramel to set completely, about 45 minutes. At this point, you can garnish further or serve. The caramel apples can be kept in the refrigerator for 1 week.