Heat the oven to 350°F/180° C
Grease a 9inch baking pan. I used 9*7 baking pan.
Melt the butter. Mix sugar, vanilla and beaten eggs in a bowl.
In another bowl mix the flour, cocoa, baking powder and salt if using.
Make a well in the centre.
In the dry ingredients add the wet ingredients and mix well.
Stir in the walnuts.
transfer the batter to the greased pan and spread it.
Bake for 25 to 30 minutes till the toothpick comes out clean.
Cool on the wire rack for 10 minutes before cutting in 8 big pieces or as you please.