Preheat the oven at 180°C. Use a bit of melted butter to grease a square 24 cm cake tin. Line it with non-stick baking paper and set aside.

Place the butter, sugar, and dark chocolate in a saucepan over low heat. Cook until melted, approximately 6-7 minutes. The mixture should be smooth.

Combine the flour and cocoa powder and sift over the chocolate mixture. Stir to combine, and add the lightly whisked eggs, 80 g of chopped pecan nuts, and vanilla essence. Stir to combine.

Pour the mixture into the pan, level with a spatula, add the remaining chopped pecan nuts on top, and bake for 45 minutes.

To see if it's ready, insert a toothpick or skewer in the centre. If it's wet, it needs more time. If there are crumbs clinging, it's ready. Let it cool slightly and cut into 12 pieces.