• Preheat the oven to 350 F (180 C) for conventional oven. For a convection oven
• Oil the 8 x 8 inch pan and line with parchment paper sling, overhanging on the sides

Prepare the batter:

• Combine butter, chocolate, brown sugar, and sugar in a heat-proof bowl set over a saucepan with simmering water. Make sure the water isn't touching the bottom of the bowl. Stir until the butter and sugar melt. Add preserved lemon paste and lemon zest and stir to combine
  113 g unsalted butter, 113 g semi-sweet chocolate, 110 g light brown sugar, 100 g sugar
  
If you only have whole preserved lemons, discard the seeds and finely chop or blend both the peel and the flesh into a paste before measuring out the 3 tablespoons.
• Add the eggs and salt (if using). Stir until well combined
  100 g eggs, 1/4 tsp salt
  
• Fold in the flour and chopped walnuts. Fold until just combined
  64 g all-purpose flour, 60 g toasted walnuts
  

Baking:

• Scrape the batter into the pan and smooth it out with a spatula to make sure it is all even
  
• Bake in the middle rack for 35-40 minutes or until the top is shiny and beginning to crack on top. Do not overbake.
  
A toothpick inserted into the center should come out with a few moist crumbs attached, not wet batter, and not completely clean.
• Set the pan on a cooling rack to let it cool down completely.

To serve:

• This brownie tastes the best the next day if you can wait.

How to store:

• Let them cool down completely and then keep in an air-tight container for 2-3 days. They freeze really well too. I cut into serving size and wrap individually with cling wrap and store in a freezer bag. Simply thaw at room temperature before serving