Wash and thoroughly dry the apples. Remove the stems by twisting them until they pop off. Insert a treat stick into the top of each apple where the stem was removed. Set aside.
In a large, heavy-bottomed saucepan, combine the butter, brown sugar, sweetened condensed milk, and corn syrup over medium-low heat. Stir until everything is melted and well combined.
Increase the heat to medium and bring the mixture to a rolling boil, stirring constantly.
*It is very important you stir constantly to avoid burning the caramel. Continue stirring constantly until the caramel reaches 240°F. Immediately remove the saucepan from the heat.
Add the large marshmallows to the caramel mixture. Slowly stir and fold them in until they are completely melted and incorporated.
Dip each apple into the caramel, tilting the saucepan partially on its side and slowly twisting the apple until it is completely coated. Gently shake and twirl each apple over the saucepan to let any excess caramel drip off.
Scrape the bottom of each apple against the side of the saucepan before placing it on a silicone-lined baking sheet. Now, add your favorite toppings! The toppings stick best when you roll the apples in the toppings while the caramel is still warm and hasn't set.
For a chocolate drizzle or a full chocolate coating, let the caramel set completely before adding the chocolate then the toppings on top of the chocolate.
Repeat process with all of the remaining apples. Allow the caramel and/or chocolate to set before serving. Enjoy!