Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a large mixing bowl, cream together cold unsalted butter, brown sugar, and granulated sugar until smooth and creamy (about 2-3 minutes).

Add one large egg and vanilla extract, mixing on low speed until just combined (about 30 seconds).

Gradually add all-purpose flour, baking soda, and salt, mixing until just combined (about 30 seconds), avoiding over-mixing.

Fold in butterscotch chips and semi-sweet chocolate chips gently to avoid tough cookies.

Using a cookie scoop, drop generous amounts of dough onto prepared baking sheets, leaving space between each mound.

Optionally, place extra chips on top of each cookie dough mound and lightly press down.

Bake for 8-10 minutes for medium cookies or 10-13 minutes for larger cookies until edges are lightly browned.

Let the cookies cool on the baking sheet for about 5 minutes before transferring to wire racks.

While still warm, sprinkle sea salt over the tops of the cookies.