Preheat the oven to 375°F (190°C). Line a rimmed baking sheet with parchment paper or foil and spray lightly with nonstick spray.
Spread pretzels in a single layer across the entire sheet. Break them up slightly if needed to cover any gaps.
In a saucepan over medium heat, melt butter and stir in brown sugar. Bring to a boil, then simmer for 3–4 minutes, stirring constantly until thick and bubbly.
Pour the toffee mixture evenly over the pretzels and spread it with a spatula.
Bake for 5 minutes, then remove and immediately sprinkle chocolate chips evenly over the hot toffee.
Let sit 5 minutes so the chocolate melts, then spread it into an even layer. Sprinkle with flaky sea salt, if using.
Cool completely, then refrigerate for 1 hour or until firm. Break into pieces and enjoy!