Preheat the oven to 350 degrees F.
Coat an 8-inch square baking dish with cooking spray.
In a large mixing bowl or in the bowl of a standing mixer, combine the butter or margarine, brown sugar and vanilla. Mix on high for one minute, until light and fluffy.
Add the eggs, mixing on low until fully incorporated; then increase the speed to high and continue mixing for two minutes, until light and fluffy.
Reduce the speed to low, add the flour, and continue mixing until fully incorporated.
Fold in the nuts and chocolate with a spoon, until just blended.
Spread the batter in the prepared pan and bake for 35 minutes or until a toothpick inserted in the center comes out clean.
Let cool in the pan on a wire rack for at least 30 minutes.
Cut into 16 squares.