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

In a large mixing bowl, cream together the chilled butter, brown sugar, and granulated sugar until smooth and creamy.

Add the egg and vanilla extract to the mixture, then mix on low until combined.

Gradually mix in the flour, baking soda, and salt until just incorporated.

Fold in the butterscotch and semisweet chocolate chips gently with a spatula.

Scoop the dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake in the preheated oven for 8-13 minutes until lightly browned around the edges.

Let the cookies cool on the baking sheets for 5 minutes before sprinkling with sea salt and transferring to a wire rack.