Preheat the oven to 180°C (356°F).
Place dark chocolate and butter in a bowl and melt them together using a double boiler.
Add sugar and eggs one at a time, mixing well after each addition.
Sift cake flour, cocoa powder, and salt into the mixture and gently combine.
Add desired toppings such as nuts or chocolate chips.
Pour the batter into a pan lined with parchment paper and spread it evenly.
Bake in a 180°C (356°F) oven for 25–30 minutes.
Let cool, then cut according to desired portions.