Preheat the oven to 200°C (392°F).
In a bowl, combine plain yogurt, eggs, and sugar and mix well.
Sift in cake flour and cocoa powder, and mix until well combined.
Add melted butter and dark chocolate, and mix thoroughly.
Pour the batter into a round pan lined with baking paper.
Smooth the top of the batter and bake in the preheated oven for 30 minutes.
Remove from the oven, let it cool, then refrigerate for at least 2 hours until set.