In a large bowl, whisk together the melted butter and brown sugar until smooth and combined, about 30 seconds. 

Next, add the egg and vanilla and whisk to combine. 

Add the flour and baking soda and whisk just until no large lumps of flour remain. Be careful to not over mix. 

Add the butterscotch chips and chocolate chunks and fold them gently into the cookie dough. 

Transfer the batter to the fridge and allow it to chill uncovered for 30 minutes. 

While the dough chills, preheat your oven to 350 degrees F and line two large baking sheets with parchment paper or Silpat mats. 

Scoop one-tablespoon-sized cookies onto the baking sheet, making sure to scoop them at least two inches apart as these cookies will spread. 

Bake for 12-14 minutes, or until the cookies are golden brown and set around the edges. 

Allow the cookies to cool fully on the tray, they will set up as they cool. 

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