Preheat the oven to 350°F. Have ready two ungreased baking sheets.
Mix the flour, baking soda and salt together in a bowl and set aside.
In a large mixing bowl using a hand held electric mixer, beat the butter and both sugars until well mixed. Gradually add the oil and beat for another minute, scraping sides of bowl often. Add the egg and continue beating on high for about a minute, and then beat in the vanilla. With a large mixing spoon, gradually add the flour mixture and stir until incorporated. Stir in the butterscotch chips and cashews
Scoop up dough in packed, level tablespoons and shape into balls. Arrange on ungreased baking sheets, spacing 2 ½ inches apart, and bake one sheet at a time for 10-12 minutes or until cookies start to brown and appear set. Remove from oven and let cool on baking sheets for about 5 minutes. Carefully transfer to a wire rack to cool. Let cookies cool completely before serving (they crisp as they cool).
Makes about 28 cookies