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

In a large mixing bowl, cream the cold unsalted butter with brown and granulated sugars until smooth.

Add the egg and vanilla extract, mixing until just incorporated.

Whisk together the flour, baking soda, and salt in a separate bowl, then add to the wet ingredients.

Fold in butterscotch chips and semisweet chocolate chips without overmixing.

Scoop rounded portions of dough onto the prepared baking sheets, about 2 inches apart.

Press additional chips into the tops of the dough mounds before baking.

Bake cookies for 10-13 minutes until edges are lightly browned.

Cool on baking sheet for 5 minutes before transferring to a wire rack.

Sprinkle a pinch of sea salt over each cookie while warm.

Transfer cookies to a wire rack to cool completely.