Preheat oven to 180ºC (356ºF) and line a square 9-inch x 9-inch baking pan with parchment paper, and set aside.
In a mixing bowl, combine flour and cocoa powder, whisk together and set aside.
Prepare a double boiler by placing a heat-proof mixing bowl over a small/medium-sized pot of simmering water.
Add broken-up chocolate and butter to the mixing bowl and gently melt over low-medium heat until completely melted together and smooth.
Turn off the heat and remove the bowl from the pot.
Add the sugar and vigorously whisk through until well combined. The sugar won’t completely dissolve.
Set the chocolate mixture aside for 10-15 minutes to cool until it’s just warm to the touch. If it’s too hot, when you add the eggs next, they will curdle/cook.
Pour in lightly whisked eggs, whisking until well combined.
Add dry flour cocoa mixture and gently fold using a spatula or wooden spoon until incorporated, ensure not over-mix. It’s ok if there are still only some small lumps of flour in the batter.
Pour batter into the lined baking pan and smooth out the top.
Roughly chop the nuts and sprinkle them on top of the brownie.
Bake for 20-23 minutes or until a skewer inserted into the middle comes out with only a few moist crumbs. It will look slightly under-baked which is what you want to achieve fudgy brownies.
Remove and cool in pan for 15 minutes.
Transfer to a wire rack to cool completely.
Cut into squares and enjoy!