Preheat the oven to 250°F (120°C). Line a large rimmed baking sheet with parchment and spread the pretzels evenly over it.
Melt the butter in a saucepan. Stir in brown sugar, corn syrup, and salt. Bring to a gentle boil and cook for 3–4 minutes, stirring constantly, until smooth.
Remove from the heat. Stir in vanilla and baking soda; the mixture will foam. Pour the warm toffee over the pretzels and toss gently until coated.
Bake for 45–60 minutes, stirring every 15 minutes. Watch closely near the end so the coating does not burn.
Spread the pretzels in one layer and cool completely, about 30 minutes. Break up clusters. If using chocolate, drizzle it over the cooled pretzels and add nuts. Let the topping set before serving.