Preheat the oven to 180°C (356°F).
Mash the avocado until smooth.
Add eggs and sugar to the mashed avocado and mix well.
Melt dark chocolate and butter together over a double boiler, then add to the batter.
Sift cocoa powder, cake flour, and salt, then add to the batter and mix.
Pour the batter into a baking pan and spread evenly.
Bake in the preheated oven for 20–25 minutes.
Let it cool, then cut into desired sizes.