1. Preheat oven to 350°F (175°C). Line a 9×13-inch baking dish with parchment paper leaving an overhang for easy removal, or use a foil-lined baking sheet sprayed with nonstick spray.
2. Spread crushed pretzels evenly in a single layer on the bottom of the prepared baking dish.
3. In a medium saucepan, melt butter over medium heat.
4. Stir in the brown sugar and cook, stirring constantly, until the mixture is smooth and bubbly, about 2-3 minutes.
5. Remove from heat and stir in vanilla extract if using.
6. Pour the caramel mixture evenly over the pretzels, spreading to cover completely.
7. Bake for 10-12 minutes until caramel is bubbly and golden but not burnt.
8. Remove from oven and immediately sprinkle chocolate chips evenly over the hot caramel layer.
9. Let sit a few minutes to soften, then spread chocolate into an even layer using a spatula.
10. Sprinkle sea salt on top if desired.
11. Allow bars to cool completely at room temperature or refrigerate until the chocolate is fully set, about 30 minutes to 1 hour.
12. Use the parchment overhang to lift bars out of the pan and cut into squares or break into pieces.