Preheat oven to 350ºF. Line a 9x13 inch pan with parchment paper and spray with nonstick spray. Set aside.
Prepare brownie batter according to directions on the box. Pour into pan and bake according to box directions.
Meanwhile, place Nutter Butter cookies into a food processor. Pulse until finely crushed. Add cream cheese and pulse until fully blended. It should resemble a dough-like consistency.
Once brownie is cooled, spread the Nutter Butter-cream cheese mixture over the brownie. (I found it easiest to spoon dollops of the mixture evenly over the brownie, and then gently pressing it down to form an even layer.) Store in the refrigerator while you prepare the ganache.
Pour heavy cream into a microwave-safe bowl and add the butter. Heat in a microwave until it begins to boil. My microwave took about 1 minute and 30 seconds. Remove from microwave and add chocolate. Cover bowl with plastic wrap and let sit for 5 minutes. Remove wrap and stir until smooth.
Remove brownies from the refrigerator, pour ganache over it and spread evenly with a knife or offset spatula.
Return to the refrigerator for at least 1 hour to allow ganache to set. Once the ganache is set, cut into 24 squares and serve. Brownies stay fresh in the refrigerator for up to 1 week.