PREHEAT OVEN TO 160ºC . Melt the chocolate in the top of a double boiler. Cut the butter into tablespoon pieces, place in the bowl of a mixer and mix until smooth. Slowly add the melted chocolate until incorporated. Add the eggs, one at a time, as each one is absorbed. Add the sugar, then the flour. Lightly grease a 9-by-11-inch baking pan. Pour in the batter and smooth the top. Place in the oven and bake for 25 to 30 minutes. When the cake is cooked, remove from the oven and let cool before removing from the baking pan. To serve, cut the cake into individual portions and top with fresh raspberries. Accompany with whipped cream or raspberry sauce, if desired.