From Anne's Food

 

200 g dark chocolate
125 g unsalted butter, at room temperature
280 g sugar
1 small vanilla bean, seeds
2 eggs
150 ml flour
2 tbsp cocoa powder

Finely chop the chocolate. Set aside half of it. Melt the other half. Beat butter and sugar until fluffy, and add the seeds from the vanilla bean. (Feel free to use extract if you prefer.) Mix in one egg at a time. Stir in the melted chocolate, and then the flour and cocoa powder. Finally add the rest of the chopped chocolate.

The batter will be fairly stiff. Scrape it into a buttered pan (mine is about 20 cm square) and bake at 175°C for about 25-30 minutes.