Preheat the oven to 350 F and line two baking sheets with parchment paper.
Beat the butter, brown sugar, and granulated sugar until light and creamy. Mix in the egg and vanilla until smooth.
Whisk the flour, baking soda, and fine salt in a separate bowl. Add the dry ingredients to the butter mixture and mix just until combined.
Fold in three-quarters of the crushed pretzels, the toffee bits, and the dark chocolate chips. Refrigerate the dough for 20 minutes.
Divide the chilled dough into 12 portions and arrange them at least 2 inches apart on the prepared baking sheets. Press the remaining pretzel pieces onto the tops.
Bake for 10 to 12 minutes, until the edges are golden and the centers remain soft. Sprinkle with flaky sea salt and cool on the baking sheets for 10 minutes before transferring to a cooling rack.