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

In a large mixing bowl, cream together cold unsalted butter, brown sugar, and granulated sugar using an electric mixer for 2-3 minutes.

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

Gradually sift in all-purpose flour, baking soda, and salt, mixing until just combined (15-20 seconds).

Fold in butterscotch chips and semisweet chocolate chips until evenly distributed.

Using a cookie scoop or spoon, drop mounds of dough onto the prepared baking sheets, spaced about 2 inches apart.

Bake for 8-13 minutes, watching for edges to be golden brown while centers remain soft.

Let the cookies cool on the baking sheets for 5 minutes and then transfer to wire racks. Sprinkle with sea salt before serving.