Make one of the boxes of brownie mix according to directions, and spread into a 9" x 12" cake pan that has been lined with parchment and sprayed with non-stick spray. Bake until just done, but do not add the glaze topping packet. When cool, turn out onto a rectangular platter and put in freezer.
In same pan, make a second batch of brownies. Cool thoroughly.
Soften the ice cream and spread in a very thick layer (about 1" on first cake. Working quickly, flip the second cake on top and align.
Empty both fudge glaze packets onto top of dessert, and sprinkle the hazelnuts on top. Return to freezer until ready to serve.