Preheat oven to 350F.
Grease and flour a 9x13 inch baking dish.
In a medium-sized bowl, sift together the flour, baking powder, baking soda, and salt.
In a large bowl, whisk together the sugar and oil until light and fluffy.
Whisk in the cocoa powder until combined.
Beat in the eggs one at a time, and then stir in the vanilla.
Add the buttermilk and mayonnaise, and stir with a whisk until fully combined.
Add the flour mixture you combined earlier to the wet mixture, until the batter is evenly mixed.
Stir in the chocolate chips, and walnuts and pecans.
Pour the cake batter into the prepared pan.
Bake in the oven at 350F for 25 to 30 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Cool in the pan for 30 minutes, and then turn out onto a wire rack and allow to cool completely.
Spread the chocolate frosting over the chocolate cake.
Sprinkle the chocolate shavings over the top of the cake.
Cut cake into slices, and serve with cold glasses of milk.