Brownie Ice Cream Bars

Preheat the oven to 350F.
Line a 9 x 13-inch baking dish with parchment paper; set aside.
Melt 1 1/2 sticks butter and 6 ounces chocolate in a heatproof bowl set over a pan of simmering water, stirring; set aside.
Whisk together the eggs, salt, sugars, and vanilla in a large bowl.
Stir in the chocolate mixture; fold in the flour.
Pour the batter into the lined baking dish.
Bake until the top is shiny, about 20 minutes.
Let cool in the dish on a wire rack.
Cover; let stand 8 hours (or overnight).
Run a thin knife around the edges of the brownie; invert to unmold onto a cutting board.
Remove the parchment.
Trim 1/4 inch from the edges of the brownie.
Line the baking dish with a clean sheet of parchment paper, leaving a 3-inch overhang on the long sides.
Return the brownie to the dish, top side up; set aside.
Mix half the ice cream in the bowl of an electric mixer fitted with the paddle attachment until smooth but not melted; spread over the brownie.
Cover; freeze 1 hour.
Mix the remaining ice cream, and spread over the frozen layer of ice cream on the brownie.
Cover; freeze 1 hour (or overnight).
Melt the remaining 40 ounces chocolate with the remaining 2 tablespoons butter.
Let cool.
Transfer the brownie to a cutting board.
Dip a knife in hot water; wipe dry.
Trim 1/4 inch from the edges.
Cut the brownie into 12 bars, dipping the knife as needed.
Transfer to a wire rack set over a baking sheet.
Ladle the chocolate mixture over the bars.
Freeze 30 minutes.