Brownies:
6 ounces unsweetened chocolate, chopped
1 1/2 sticks (3/4 cup) unsalted butter, cut into pieces
2 cups granulated sugar
3 large eggs
1 cup all-purpose flour

Salted Caramel Sauce:
1 cup packed brown sugar
1/2 cup heavy cream
4 tablespoons (1/2 stick) salted butter
1 tablespoon vanilla
1 teaspoon kosher salt

Salted Caramel Buttercream:
4 cups confectioners' sugar
2 sticks (1 cup) salted butter
1/4 cup heavy cream