Preheat oven to 350 degrees F. Spray a 9x13- inch baking dish with nonstick cooking spray; set aside.
In a large mixing bowl, mix together all brownie ingredients until well combined. Spread batter in an even layer in prepared pan.
Bake for 20-22 minutes, or until an inserted toothpick comes out clean and let cool completely. Cut brownies into bite-sized pieces; set aside.
In a large mixing bowl, mix together all mousse ingredients until well combined; set aside.
Layer 1⁄3 the brownie pieces, 1⁄3 peanut butter mousse and repeat layers until trifle bowl is filled. Top with Reeses; cover and refrigerate until serving.