For the brownies: Preheat the oven to 325 degrees F. Line a 9-by-13-inch baking pan with foil or parchment paper.
In the top of a double boiler over medium heat, heat the chocolate and butter, stirring, until melted, about 5 minutes. Add the granulated sugar and stir gently until combined. Remove from the heat.
In a separate bowl, gently whisk the eggs. Add a small splash of the chocolate mixture to the eggs, stirring immediately (but still gently). Then add the tempered eggs into the chocolate, stirring constantly. Add the flour and stir until combined. Pour the batter into the prepared baking pan and spread to even out the surface.
Bake until just beginning to crisp at the edges, 25 to 30 minutes. Let cool completely in the pan.
For the salted caramel sauce: Add the brown sugar, cream, butter, vanilla and salt to a pan and cook over medium heat, whisking occasionally, until thickened and combined, about 5 minutes. Remove from the heat and allow to cool.
For the salted caramel buttercream: Add the confectioners' sugar, butter and cream to a mixer fitted with the paddle attachment. Mix until light and fluffy, 3 to 4 minutes. Turn the mixer off, drizzle in the salted caramel sauce, turn the mixer back on and mix until combined.
To frost the brownies: Remove the cooled brownies from the pan, spread with the buttercream and cut into individual pieces.