Step 1

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.

Step 2

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

Step 3

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.

Step 4

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.

Step 5

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

Step 6

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

Step 7

Bake it for 33 to 35 minutes.

Step 8

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

Step 9

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