Pre-heat your oven to 350°F (180°C). Line the sides and bottom of an 11” x 7” (28cm x 18cm) rectangular baking pan with overlapping pieces of parchment paper and set aside.
Melt the chocolate and butter together over a gently simmering double boiler, stirring often. Remove from the heat. Whisk together the egg yolks and sugar until smooth and pale yellow. Stir the yolks, flour and ground almonds into the chocolate until evenly blended.
Whip the egg whites to soft peaks and gently fold them into the chocolate in three stages, until just blended. Pour the batter into the prepared pan and bake for 25-30 minutes.
Let cool and carefully remove from the pan using the parchment paper handles, cut into desired shape. Serve warm, garnishing with a scoop of ice cream, drizzler and fresh berries if desired.