Prepare brownie mix according to manufacturer’s directions, adding in 5% whole eggs. Deposit 5 pounds batter into half sheet trays with extended sides that have been lined with parchment and sprayed. Bake until set all the way through, let cool completely.
In a stand mixer fitted with the paddle attachment, place 10 pounds buttercreme base and 3 pounds 2oz chocolate hazelnut spread. Mix with a paddle on low speed to soften. Slowly stream in water while mixing. Scrape down the bowl, then mix for 7-10 minutes on low speed. Reserve. Reconstitute another 10 pounds buttercreme base according to manufacturer’s directions. Pipe prepared buttercream onto brownies, then use a cake comb to smooth. Sprinkle with hazelnuts and chill overnight before slicing.