Preheat your oven to 350°F / 175°C.
Grease an 8-inch or 9-inch square baking pan, or for easy removal, line it with parchment paper or foil and then grease it.
In a small saucepan, gently melt the 8 tablespoons (120 ml / 4.1 fl oz) of butter and the unsweetened chocolate over low heat, stirring frequently until smooth.
Remove from heat.
Transfer the melted chocolate mixture to a mixing bowl.
Using an electric mixer, beat in the sugar until the mixture is smooth and glossy.
Beat in the egg until well combined, scraping down the sides of the bowl as needed.
Add the salt, then gently stir in the flour with a spatula until just combined—be careful not to overmix.
Fold in the chocolate chunks and the nuts, if using.
Pour the batter into your prepared pan and spread it into an even layer.
Bake for 20 to 25 minutes, or until the center is just barely set. A toothpick inserted should come out with moist crumbs, not wet batter. If the edges are baking too quickly, rotate the pan halfway through.
Let the bars cool completely on a wire rack before cutting into squares. Store in an airtight container at room temperature for up to 2 days.
