In the bowl of a stand mixer fitted with the paddle attachment, beat together the softened butter, white sugar, and brown sugar on medium-high speed for about 1 minute, until smooth and creamy.

Add the egg and vanilla extract, beating on medium-high speed for an additional 1 minute until the mixture is light and fluffy.

Gradually add the dry ingredients to the wet ingredients, mixing on medium speed until just combined.

Add the chopped pretzels, toffee bits, and chocolate chips to the dough, and stir until evenly distributed.

Transfer the dough to an airtight container and chill for at least 1 hour (up to 2 days).

Preheat your oven to 375°F (190°C). Line two large baking sheets with silicone baking mats or parchment paper.

Using a 2-tablespoon cookie scoop, portion the dough into balls and place them on the prepared baking sheets, spacing them about 2 inches apart. Press extra toffee bits, chocolate chips, and pretzel pieces on top for extra decoration if desired.

Bake the cookies in the preheated oven for 9 to 11 minutes, or until the edges are golden brown, and the tops are just set.

Remove the cookies from the oven and immediately sprinkle them with flaky sea salt. Let the cookies cool on the baking sheet for 5-10 minutes.

Transfer the cookies to a wire rack to cool completely.