Line two baking sheets with parchment paper or silicone baking mats; set aside.
In the bowl of an electric mixer fitted with the paddle attachment (or in a large bowl using a hand mixer), beat the melted unsalted butter and packed brown sugar until smooth and combined.
Add the vanilla and egg; beat until the mixture is smooth.
Add the dry instant butterscotch pudding mix (the powder from the box) and mix until incorporated.
Add the salt and baking soda, then add the all-purpose flour and mix until the dough is well blended and no dry streaks remain.
Let the dough rest for 5 minutes, then stir in 1 cup of the butterscotch chips and the 1/2 cup chopped cashews or peanuts.
Use a 2-tablespoon cookie scoop or spoon to drop dough balls onto the prepared baking sheets, spacing them about 2 inches apart. Very gently press each ball down slightly with the palm of your hand.
Press the remaining butterscotch chips (about 1/3 cup) onto the top of each cookie (and extra cashews if you like) for decoration.
Chill the cookie sheets with the dough in the refrigerator for at least 30 minutes.
Preheat the oven to 350°F (177°C) while the dough chills (or immediately after removing from the fridge).
Bake the cookies on a middle rack for 11–14 minutes, until the edges lose their sheen (11–12 minutes for softer centers; bake longer if you prefer firmer cookies).
Remove the baking sheet from the oven and let the cookies cool on the sheet for 2–3 minutes, then transfer the cookies to a wire rack to cool completely.