Preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with heavy foil, extending foil over the pan edges. Lightly grease foil. Set pan aside. In a large bowl, beat butter with an electric mixer on medium to 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 graham cracker crumbs.

Reserve 1 cup of the oat mixture. Spread remaining oat mixture into prepared pan.* Bake in the preheated oven about 15 minutes or until lightly browned. Sprinkle marshmallows evenly over warm crust. Top with chocolate chunks. Dot remaining oat mixture atop chocolate and marshmallows.

Bake 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. Makes 24 bars.

Wet the back of a large spoon and use to spread oat mixture in pan.

Place bars in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months.