Place the oven rack in the middle position and heat the oven to 325 F degrees. Line a 13X9 inch baking pan with aluminum foil. Spray it with vegetable oil spray.

Mix the flour, baking powder and salt in a bowl and set aside.

Using the bain-marie double boiler method (place a heatproof bowl over barely simmering water), melt the chocolate and butter. Stir it occasionally, until it becomes smooth.

Off the heat, transfer the now-melted chocolate to a large glass bowl. Whisk in the sugar into the chocolate-butter mixture in three batches. Add eggs one at a time, whisking after each addition, until they are thoroughly combined. Whisk in the vanilla extract.

Fold in the flour mixture in three additions, until it is completely smooth and thoroughly combined.

Pour the batter into the prepared baking pan. Using a spatula, spread it evenly and into the corners. Sprinkle it with walnuts.

Bake it for 33 to 35 minutes.

Cool on a wire rack for an hour before cutting it into small squares.

Serve it with vanilla ice cream or with a cup of cold milk.