Step 1

Coat a 9- x 13-inch baking dish with cooking spray.

Step 2

In a soup pot over low heat, melt 1/2 stick butter. Add marshmallows and stir constantly until melted. Remove from heat and stir in cereal; mix until evenly coated. Spoon mixture into baking dish and evenly press down.

Step 3

In a small saucepan over medium-low heat, melt remaining butter. Add condensed milk, corn syrup, vanilla, and brown sugar; cook 5 to 8 minutes, or until thickened and golden in color, stirring constantly. Evenly pour over cereal mixture; set aside 15 minutes.

Step 4

In a small microwaveable bowl, combine chocolate chips and shortening. Microwave 60 to 90 seconds, or until smooth, stirring occasionally. Gently spread over caramel mixture. Let cool 1 hour, or until firm, then cut into bars.