1. Coat a 15 x 10 x 1-inch or 13 x 9 x 2-inch pan with nonstick cooking spray. Melt butter in a large pot over medium-low heat. Add marshmallows and cook until melted and smooth, about 5 minutes.
2. Remove pot from heat and stir in Cheerios and raisins. Let stand 2 minutes (mixture will stick to pot if you wait too long).
3. Fold in butterscotch morsels until well distributed. With greased hands, gently press mixture into prepared dish.
4. Let stand at room temperature until cool. Cut into 24 bars.