Grease an 8 x 8-inch (20 x 20-centimeter) square baking pan and line with parchment paper.
Melt the chopped chocolate and butter. To melt the chocolate, finely chop the chocolate and melt in a heat proof bowl over barely simmering water or in a microwave oven on medium-high power (30-45 second intervals, stirring in between).
Remove from the heat and let cool slightly.
Beat together the eggs, sugar, vanilla, and salt until well combined.
Add the melted chocolate, and beat gently by hand or on a low setting on a mixer until evenly incorporated.
Add the flour and walnuts, and mix until just combined.
Pour the mixture into the prepared pan and bake approximately 20 minutes.