Preheat the oven to 350 degrees F.
Line a 10 by 13-inch pan with parchment paper set aside. Bring a small saucepan of water to a boil, and remove from heat. Combine 1 1/2 sticks of butter and 6 ounces of chocolate in a small heatproof bowl; set over saucepan. Let stand, stirring occasionally, until chocolate is melted and smooth.
Whisk the eggs together in a large bowl; add salt, sugars, and vanilla; stir to combine. Stir in the butter and chocolate mixture, then fold in flour. Pour the batter into prepared pan, spreading evenly. Bake until top is shiny, about 20 minutes. Do not over bake; a cake tester should not come out clean.
Let cool completely on a wire rack. Store in an airtight container overnight. The next day, loosen corners of brownie. Invert onto a cutting board, and remove parchment. Trim about 1/4-inch from each edge.
Place a sheet of parchment paper in same baking pan, allowing excess to overhang the edges of pan. Return brownie to pan, top side up; set aside.
Meanwhile, working quickly, scoop 2 pints of the ice cream into the bowl of an electric mixer fitted with the paddle attachment. Beat the ice cream until soft and smooth, but not melted. Using a rubber spatula, transfer the ice cream to brownie pan, spread evenly. Transfer to freezer. Repeat process with the remaining 2 pints of ice cream, adding a second layer of ice cream. Freeze at least 1 hour and up to overnight.
Bring a medium saucepan with 2 inches of water to a boil. Remove from heat. Place the remaining 40 ounces of chocolate and 2 tablespoons butter in a heatproof and set over water. Stir until melted and smooth.
Set a wire rack over a baking sheet; set aside. Remove pan from freezer Using the edges of the parchment paper, remove brownie from pan, and place on a cutting board. Dip a sharp knife in hot water, wiping to remove excess. Working quickly, trim 1/4-inch from edges. Cut brownie into 12 (2 1/4 by 4 1/4-inch) bars, and place on prepared wire rack.
Ladle chocolate and butter mixture over bars, working from front to back. Cover as much of the ice cream as you can. Freeze until firm, about 30 minutes. The bars may be stored in the freezer, in an airtight container for up to 1 week.
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