Add sweetened condensed milk and chocolate chips to a saucepan. Heat over medium heat, stirring constantly, until chocolate chips are melted.
Remove from heat and stir in butter and vanilla. Stir until butter is melted.
Place hot fudge in a jar. Use while hot.
Store leftovers covered in the refrigerator for up to 2 weeks. To reheat: place jar in microwave and heat in 30 second increments, stirring between each, until melted and smooth.
