Preheat your oven to 350°F and line two baking sheets with parchment paper. In a large bowl, use an electric mixer on medium speed to beat the butter and sugars together until smooth and fully combined, with no lumps of butter remaining. Add the egg and vanilla extract, mixing on low speed until just blended. In a separate bowl, combine the flour, baking soda, and salt, then gradually add these dry ingredients to the wet mixture, mixing only until just incorporated—avoid overmixing the dough. Fold in the butterscotch chips, chocolate chips, and any optional add-ins, mixing gently until evenly distributed. Using a small or large ice cream scoop or a heaping tablespoon, drop the dough onto the prepared baking sheets, spacing 6 to 8 cookies per sheet depending on your desired cookie size. If you like, press additional butterscotch and chocolate chips gently onto the tops before or after baking. Bake one sheet at a time until the bottoms and edges turn lightly golden and the tops feel firm when touched gently. For medium-sized cookies (about 2 tablespoons dough each), bake for 8-10 minutes; for larger cookies (about ¼ cup dough), bake for 10-13 minutes. Allow the cookies to cool on the baking sheets for 5 minutes, then transfer them to wire racks to cool completely. While cooling, sprinkle the cookies with sea salt to achieve that perfect sweet-and-salty balance. This recipe yields between 12 and 30 cookies, depending on the size you choose.
