In the top of a double boiler (or a bowl set on top of a pot of simmering water), melt the butter and chocolate in the top of a double boiler, whisking until smooth. Cool to room temperature.
While the chocolate mixture cools, in a large bowl, whisk together sugar, eggs, and vanilla extract, set aside. In a small bowl, sift together flour and salt, set aside.
Whisk in a small amount of the chocolate mixture into the egg mixture, then slowly add the rest and whisk until combined. Gently fold in the flour mixture with a spatula.
Pour batter into the prepared pan. Bake for 20 – 30 minutes. These will seem slightly underbaked, as they are a gooey brownie. Allow to cool to room temperature. Use a knife to cut the pan of brownies down the center so you end up with 2 nearly square shaped pieces. Place pan in the freezer for about 30 to 60 minutes until the brownies are cold and partially frozen.