Grease a 9x13-inch baking dish and set aside.
In a large pot over medium heat, melt the butter.
Add the mini marshmallows and stir continuously until melted and smooth.
Remove from heat and stir in the vanilla extract and salt.
Quickly fold in the Apple Jacks cereal and mix until evenly coated.
Pour the mixture into the prepared dish and press down gently with a spatula.
Let cool completely before cutting into bars.