Preheat oven to 350° F (180° C).

In a small pot, melt the dark chocolate and butter. In a separate bowl whisk together the eggs, vanilla extract, and sugar.

Cool the chocolate mixture and mix with the egg mixture. Add in flour and baking powder (and walnuts, if desired).

Pour the batter into a greased medium size baking pan, approximately 11x13 inches (30x35cm)

Bake for 25-30 minutes until fully cooked through.