In a large bowl, whisk together the melted butter and brown sugar. 

Add the egg and vanilla extract and mix to combine. 

In a separate bowl, whisk together the all-purpose flour, baking soda, and kosher salt. Gradually add the dry ingredients to the wet ingredients, mixing just until the dry ingredients are incorporated. 

Fold in the butterscotch chips and semi-sweet chocolate chips.

Cover the dough with plastic wrap and refrigerate for 30 minutes. Chilling the dough will help prevent the cookies from spreading too much during baking.

While the cookie dough chills, preheat your oven to 350°F and line two baking sheets with parchment paper or silicone baking mats.

Once the dough has chilled, use a two tablespoon cookie scoop to scoop out the dough. Space them about 2 inches apart on the prepared baking sheets. 

Bake the cookies for 10 minutes, then remove them from the oven and tap the tray on the counter to sink the centers. Run a round cookie cutter around the edges of the cookies to pull them back toward the center– this will help the cookies stay perfectly round and prevent the edges from burning. Return the cookies to the oven and bake for another 5 minutes. 

Remove the baking sheet from the oven and let the cookies cool on the sheet for 5 minutes. Then transfer them to a wire rack to cool completely.

Store any leftover cookies in an airtight container at room temperature for up to four days.