Melt dark chocolate and butter using a double boiler.
Mix sugar and eggs until creamy.
Add melted chocolate and butter to the egg mixture and combine.
Sift cake flour and salt, then gently mix them in.
Line a pan with parchment paper and pour in the batter, smoothing it out.
Bake at 180°C for 25 minutes.
Let cool completely, then cut into bite-sized pieces.