Melt the chocolate in the microwave at 600 watts for 1-2 minutes. Thinly grate 1 tsp. zest from the orange and squeeze out 4 tbsp. juice.
Coarsely chop the nuts. Beat eggs, sugar and orange zest for at least 7 minutes until creamy. Preheat the oven to 180 °C (convection oven 160 °C).
Stir the chocolate and orange juice into the egg mixture. Mix flour, cocoa and baking powder and sift over the batter, stir in with the nuts.
Spread on a baking sheet lined with parchment paper, or pan (30×20). Bake in a hot oven for 15-20 minutes.
Let cool in the mold on a rack, sprinkle with cocoa and powdered sugar and enjoy.