Place the sugar, butter, cocoa, and salt in a large microwave-safe bowl, and microwave in one-minute increments, stirring, until the butter is completely melted and the mixture feels very warm to the touch.
Add the eggs, one at a time, stirring after each addition.
Stir in the flour.
Stir in the vanilla.
Transfer the batter to the prepared pan, and bake for 45-60 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs.
Cool completely in the pan, then frost, transfer to a cutting board to cut into squares, and garnish with chocolate curls (if using).
To make the frosting:
Place the cream cheese and butter in a large mixing bowl, and mix on medium speed until smooth.
Add the powdered sugar, one cup at a time, mixing until smooth.
When all the powdered sugar has been added, turn the speed up to medium-high and beat for one or two minutes, or until fluffy.