Line with tinfoil and lightly butter or grease a 9x13'' pan.
Popcorn Bar Mixture
Measure out popcorn, cereal and any other add-ins you want into a large mixing bowl.
Marshmallow Mixture
Place butter in a large saucepan and melt over low heat.
Once melted, add miniature marshmallows. Stir constantly until marshmallows are melted and mixture is smooth.
Pour the marshmallow mixture over the popcorn mixture and fold gently to combine well. Mixture will be very sticky.
Pour mixture into greased pan, spread out and press gently into the pan using fingertips or spatula, Don’t compact the bars too tightly. Let bars cool down before adding fudge layer.
Milk Chocolate Fudge Layer
Mix the chocolate chips and condensed milk together in a microwavable bowl and microwave for 30 seconds. Stir and repeat microwaving 30 seconds more and stir until smooth and chips are melted. If chips are not melted, microwave for additional 30 seconds, stirring and repeating until smooth. (It usually takes way less than 2 minutes.) Working quickly, pour the chocolate fudge on top of the popcorn mixture and spread out over popcorn bars. Add sea salt on top if you desire.
Place popcorn mixture in fridge for 5 minutes to set, then transfer to countertop to continue setting up 20 minutes before cutting. Pull the bars out by gripping the tin foil on sides of pan and place on cutting board, cut and serve. Seal in air-tight container for storing.
Notes
Makes 18 to 24 bars depending on how big you want to cut them. Popcorn Bars are best made not too gooey and better with more popcorn and goodies for a more crunchy/crispier texture with the fudge topping.