Cream together brown sugar, granulated sugar, salt, butter (or margarine) and baking powder.
Add and mix all-purpose flour until incorporated. Add and mix eggs until creamy. Add diced walnuts.
Mix on low with paddle for 30 seconds if using KitchenAid-style mixer. If not available, then mix with wooden spoon to incorporate, but if mixing by hand, add eggs before the flour. (A hand held mixer may not be powerful enough for this heavy batter, but try if you like.) DO NOT OVERBEAT!
Scrape down sides of bowl and then mix on medium speed for 2 minutes or by spoon for 5 minutes.
Grease two 13" x 9" x 2" pans and preheat oven to 375-400 degrees. (Note: If pan sizes are different, pour batter to a maximum of 3/4 of an inch deep.)
Divide mix evenly into pans and place on bottom rack and bake. Brownies will rise gradually for about 25 minutes and then flatten.
Wait for 3 minutes after they have flattened completely, then remove from oven and place on rack to cool.
When warm to the touch, cut to desired size.