Preheat oven to 180 degrees C. Lightly grease and flour one 13x23cm cake tin.
Combine 1/2 cup of the flour and the brown sugar, cut in the butter until the mixture is crumbly, stir in the chopped nuts and set aside.
In a double boiler or microwave melt chocolate chips and set aside to cool.
In a large bowl mix the remaining 2 cups flour, white sugar, bicarb soda and salt. Add the copha, eggs, buttermilk and vanilla. Blend and then beat with an electric mixer at medium speed for 2 minutes.
Combine 1 cup of the batter and the melted cooled chocolate, mix well.
Alternate spooning the light and the dark batters into the prepared cake tin. Cut through with a knife to marble. Sprinkle top with the reserved nut mixture.
Bake for 30 minutes or until cake tests done. Let cake cool and enjoy!