Preheat oven to 350 degrees F (175 degrees C). Grease a 12x9-inch baking dish and dust with 1 teaspoon cocoa powder.
Beat eggs, superfine sugar, margarine, flour, chocolate chunks, and 3/4 cups cocoa powder together with an electric hand mixer in a large mixing bowl until smooth. Fold raspberries gently into the batter; spoon into the prepared baking dish.
Bake in the preheated oven until a toothpick inserted into the center comes out clean and the top is a bit crusty, 40 to 45 minutes. Let cool completely before cutting.