Combine the condensed milk and chocolate chips in a microwave-safe bowl.
Microwave for 30 seconds. Stir well. Continue microwaving at 30 seconds increments until the chocolate chips are 75% melted. At that point, stir until they finish melting.
Mix in your choice of mix-ins. Transfer to the prepared pan.
Refrigerate until firm, about 3-4 hours.
Notes
Chocolate Chips - This works with any flavor chocolate chips. Mix-ins - I prefer to add roasted nuts to maximum their flavor. But raw nuts, chopped cookies, sprinkles, dried fruit, or anything else you can come up with work. If you don't want to add a mix-in use an additional half cup of chocolate chips.