Grease a 9x13-inch pan with cooking spray. Set aside.

In a large saucepan over low heat, add butter, peanut butter, and marshmallows. Stir gently until the mixture is melted. Be careful not to burn the mixture.

Grease a large bowl with cooking spray and add rice krispie cereal. Pour over melted marshmallow mixture and stir together with a spatula. Press the mixture into the bottom of the prepared 9x13-inch pan. Set aside.

Add the chocolate chips to a medium-sized microwave-safe bowl. Microwave for 45-second intervals, stirring in between each burst. Once melted, stir in vegetable oil and pour over the bars. Smooth with a spatula.

Place pan in the refrigerator and let harden for 30 minutes until the chocolate is set, but not hardened. Cut into shapes and decorate as desired. Store in an airtight container at room temperature.