Preheat oven to 350 degrees F (175 degrees C). Lightly grease and flour one 13x9 inch cake pan.
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, baking soda, and salt. Add the shortening, 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 13x9 cake pan. Cut through with a knife to marble. Sprinkle top with the reserved nut mixture.
Bake at 350 degrees F (175 degrees C) for 30 minutes or until cake tests done. Let cake cool and enjoy!
