Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine pretzels and optional pecans.
In a saucepan over medium heat, melt the butter.
Stir in the brown sugar and vanilla extract, heating until bubbling.
Remove from heat and quickly stir in baking soda and sea salt.
Pour the toffee mixture over the pretzels, and mix well until evenly coated.
Spread the coated pretzels on a parchment-lined baking sheet.
Bake for 15-20 minutes, stirring halfway through.
Allow to cool completely before serving or breaking apart.