Prepare the brownies according to the package directions. Let them cool completely before their next step.
Cookie Butter Mousse
In the bowl of a stand mixer, add in the whipping cream, cookie butter, powdered sugar and vanilla. Using the whisk attachment, mix on high until everything’s evenly incorporated and the whipping cream has thickened into a mousse-like texture, about 2-3 minutes.
Crumble the cooled brownies over and into a bowl. To the bottom of 3 small trifle bowls or glasses, add a hearty layer of crumbled brownie pieces.
Spoon a layer of cookie butter mousse over top of the crumbled brownies and use the back of the spoon to lightly spread it out evenly. Repeat one-two more times, as needed, and top with more crumbled brownie pieces.
Serve immediately, or chill until ready, and allow them to sit at room temperature a solid 5 minutes before serving.