Preheat the oven to 350 F. For easy cleanup, line your 8 inch square pan with parchment paper or foil.
Put the sugar and salt in a large mixing bowl and set aside.
Chop the chocolate into small pieces and cut up the butter. Place both in a microwaveable bowl. Microwave at 50% power for one minute, or until the butter has melted (it may take 2 to 3 minutes total, but check after each minute). Once the butter has melted, remove the bowl from the microwave and stir gently until the white chocolate has melted completely and the mixture is smooth.
Add the butter and chocolate mixture to the large mixing bowl with the sugar and salt. Stir until combined. Add the eggs and the optional Grand Marnier and stir until combined.
Sprinkle the flour over the batter and stir together until just combined. Pour the batter into the prepared pan.
Grand Marnier is optional, but even the tiny bit of it adds a very subtle sophistication to the flavor. You can also substitute the same amount of rum.