Line an 8x8" baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil generously with baking spray; set aside.
In a medium saucepan, whisk together the evaporated milk and espresso until dissolved over medium heat. Add in the chocolate chips and marshmallow fluff, stirring until chocolate and fluff are melted and the mixture is smooth. Meanwhile, while this mixture melts together, pour the Oreo cookie crumbs and powdered sugar into a large bowl.
Pour the chocolate marshmallow mixture into the bowl with the cookie crumbs and powdered sugar; toss until combined. Mixture will be thick and sticky.
Press the mixture into the prepared pan, spreading it into an even layer. I like using an off-set spatula for this! Refrigerate for 1 hour.
To make the frosting: In the bowl of a stand mixer, cream together the butter, cocoa powder and vanilla until smooth. Add in the powdered sugar and beat well, pouring in the heavy cream until frosting is light, fluffy and smooth.
Spread the frosting evenly over the brownies and top immediately with the candy sprinkles. Refrigerate for at least 2 hours or until set. Store leftovers in the fridge.