Start by lining a baking sheet with parchment paper. This will prevent the bark from sticking and make for easy cleanup later.
In a medium saucepan, combine the butter and granulated sugar over medium heat. Stir continuously until the mixture comes to a gentle boil. Allow it to boil for about 4-5 minutes without stirring until it takes on a golden caramel color.
Once the toffee reaches the desired color, remove it from heat and stir in the vanilla extract. Be cautious as the mixture will bubble up slightly.
Evenly spread the salted pretzels on your prepared baking sheet. Pour the warm toffee mixture over the pretzels, ensuring they are well-coated. Use a spatula to spread it evenly.
Using a double boiler or microwave, melt the dark chocolate until smooth. If using a microwave, heat in 30-second intervals, stirring in between to avoid burning.
Once the chocolate is melted, pour it over the toffee and pretzel mixture. Use a spatula to spread it out evenly, covering all the pretzels and toffee.
Place the baking sheet in the refrigerator for at least 1 hour, or until the chocolate is firm. Once set, remove it from the fridge.
Once the bark is set, remove it from the parchment paper and break it into irregular pieces. This gives it that rustic, homemade charm!