1Preheat oven to 350^. Line a 13x9-inch baking pan with heavy foil, extending foil over the pan edges. Light spray foil with buttery spray. Set aside pan.
In a large bowl, beat butter with mixer on medium high speed for 30 seconds. Beat in brown sugar until combined,scraping bowl occasionally. Beat in eggs and vanilla until combined. Beat in flour;stir in oats and cracker crumbs.
2Reserve 1 cup of the oat mixture. Spread remaining oat mixture into prepared pan, using the back of a large wet spoon. Bake for about 15 minutes or until lightly browned. Sprinkle marshmallows evenly over warm crust. Top with chocolate chips. Dot remaining oat mixture over top or chocolate and marshmallows.
3Bake bars about 20 minutes more or until top is lightly browned. Cool completely in pan on a wire rack. Using the edges of the foil, lift the uncut bars out of the pan. Cut into bars.