Preheat the oven to 180°C (356°F).
Melt the butter and dark chocolate together using a double boiler.
In a bowl, mix the sugar with the melted butter and chocolate.
Add the eggs and stir well.
Sift the cake flour, cocoa powder, and salt together, then mix into the batter.
Pour the batter into a pan and smooth it out.
Bake in the oven for 25 minutes.
Let it cool completely, then cut into desired serving sizes.