Preheat the oven to 350 degrees F. Grease a 10-inch by 15-inch jelly roll pan with butter. Line with parchment leaving a 2-inch overhang on the short sides of the pan. Butter the paper.

In a large bowl, combine the butter and sugar. Beat in the eggs. Stir in the salt, flour and cocoa powder until combined. Pour the batter into the prepared pan and use a spatula to spread into an even layer. Bake for 10-12 minutes until the edges of the brownie release from the pan and it feels dry to the touch. Remove the brownie from the pan using the parchment overhang and transfer to a wire rack to cool completely.

Cut the brownie in half crosswise. Line a sheet pan with plastic wrap and place one brownie half on it. Spread on the ice cream using an offset spatula. Place the other brownie half on top. Wrap in tightly plastic wrap and freeze until firm, at least 2 hours.

Cut into 12 equal rectangles.