Spray a 13 x 9 inch baking dish with cooking spray. Set aside.
In a deep large heavy bottomed pot over low heat, melt together butter and marshmallows until smooth. Stir to prevent sticking.
Add vanilla and pudding mix. Stir until combined. Continue to cook for 2-3 minutes over low heat while stirring.
Remove from heat and add rice krispies cereal. Stir with a large non stick spatula until evenly coated, work quickly.
Press mixture into prepared dish. Spray hands with cooking spray or use a piece of wax paper to press evenly into pan. Let cool completely.
To make glaze: Melt together butterscotch chips with heavy cream in a microwave safe bowl. Melt in 20 second increments stopping to stir each time. Repeat until smooth adding additional cream as needed.
Drizzle over rice krispies treats. Allow to set.
Cut into squares and enjoy. Store leftovers in an air tight container at room temperature.