Begin by preheating your oven to 350°F (175°C). Prepare a baking sheet by lining it with parchment paper.
On the lined baking sheet, spread the crushed pretzels into a single, even layer.
Place the baking sheet in the oven and toast the pretzels for approximately 5 minutes, just until they start to lightly brown. Remove from the oven and allow them to cool completely.
In a microwave-safe bowl, mix the semi-sweet and milk chocolate chips together. Heat in the microwave, using 30-second intervals, and stir after each interval until the chocolate melts and becomes silky smooth.
Once the chocolate is melted, carefully pour it over the cooled pretzels on the baking sheet. Use a spatula to spread the chocolate evenly.
Generously sprinkle the toffee bits over the melted chocolate layer. If you decide to include them, add the chopped nuts on top as well.
Lightly finish by sprinkling the sea salt flakes over the top.
Transfer the baking sheet to the refrigerator and let the bark chill for about 30 minutes, or until the chocolate has fully set.
Once set, take it out of the refrigerator and break the bark into pieces.
Notes
For a charming presentation, serve in a decorative gift box or on a colorful platter.