Preheat oven to 163 degrees Celsius. Line your baking pan with greased baking paper

Melt the butter at medium heat. Cook while whisking until it’s a pale golden brown

Remove the pan from the heat and stir in the sugar. Add the cocoa powder, sea salt, baking powder and espresso powder

Add the eggs one at a time and beat them vigorously with a wooden spoon after adding each one. When the mixture is thick, shiny and well blended add the vanilla extract and flour. Stir until the flour streaks disappear

If adding chocolate, fold it into the mixture. The mixture should be slightly warm when you do this

Spread the batter in the pan and use a knife to make light swirls in the top of the batter

Bake on the lower rack for about 25 minutes. When done and poked with a toothpick, the toothpick should come out clean