Line a 9x13 inch baking pan with parchment paper, leaving some overhang on the sides for easy removal.
Spread the crushed pretzels evenly across the bottom of the prepared pan to create a solid base.
In a medium saucepan, combine the sweetened condensed milk, butter and brown sugar. Heat over medium heat, stirring continuously, until the mixture is smooth and begins to bubble. Continue cooking for 3-5 minutes, or until it thickens slightly.
Remove the caramel mixture from heat and pour it evenly over the crushed pretzels, spreading it with a spatula to ensure even coverage.
Pour the melted chocolate over the caramel layer, using a spatula to spread it out evenly. For an extra touch, sprinkle sea salt flakes on top.
Refrigerate the bars for at least 2 hours, or until the chocolate is firm and the layers are set.
Once set, use the parchment paper overhang to lift the bars out of the pan. Cut into squares and serve.
For cleaner slices: Use a warm knife to cut the bars, wiping the blade between cuts.
Storage tip: Store bars in an airtight container in the refrigerator for up to a week.