Bring stout to boil in a small saucepan over high heat; reduce heat to medium-low. Simmer, uncovered, for 30 to 35 minutes or until reduced to 1¼ cups. Remove from heat; whisk in agave nectar. Set stout syrup aside to cool.

Preheat oven to 350°. Spray bottom and sides of a 13x9-in. glass baking dish with nonstick spray. Sprinkle about 1 Tbsp. flour on inside of prepared baking dish. Tilt and shake baking dish to coat bottom and sides with flour; remove excess flour by turning baking dish over and tapping the dish exterior. Set baking dish aside.

Set aside ¼ cup stout syrup. Beat oil, eggs and remaining stout syrup in a large mixing bowl with an electric mixer on low for 30 seconds or until combined. Add cake mix; beat on low for 30 seconds more or until moistened. Beat on medium for 2 minutes more, scraping side of bowl occasionally. Pour into prepared baking dish.

Bake for 33 to 35 minutes or until toothpick inserted near center comes out clean. Cool in baking dish for 10 minutes. Invert to remove cake from baking dish; cool cake completely on a wire rack.

Brush top of cake with reserved ¼ cup stout syrup; spread with frosting. Top with popcorn and M&M’s.