Prepare pan by lining an 11 x 7-inch baking dish with foil, allowing excess foil to hang over pan edges. If you want the treats thinner, use a 9 x 13 - for thicker treats still, use an 8 x 8-inch pan.

Grease the foil with nonstick cooking spray. Melt butter in a large pot over low heat. Add marshmallows, semisweet chips, and salt and cook, stirring constantly, until melted and smooth, about 8 minutes. Stir in vanilla. Off heat, stir in rice krispies, caramels, and pecans until incorporated. Scrape mixture into prepared pan and press into bottom and corners with a greased spatula.

Let cool completely, about 1 hour. Melt semisweet chocolate chips in the microwave on low at 30 second intervals until smooth. Pour melted chocolate into a ziploc baggie, pressing chocolate into one corner of the bag.

Using scissors, clip a tiny piece off the corner and drizzle chocolate over cooled treats. Allow chocolate to cool, about 45 minutes. Using foil overhang, lift treats from pan. Cut into squares. Serve.