Whole Wheat Zucchini Chocolate Cake

1/2 cup yogurt
1/4 cup vegetable oil
1 cup sugar
2 eggs
1 teaspoon vanilla
1/2 cup buttermilk or milk soured with 2 teaspoons vinegar
1 teaspoon salt
2 1/2 cups whole wheat flour
4 tablespoons cocoa
1/2 teaspoon baking powder
1 teaspoon baking soda
2 cups finely diced, peeled zucchini
1/2 cup chocolate chips
Powdered sugar icing if desired

Stir yogurt and oil with sugar, then add eggs, vanilla, and buttermilk. Sift together flour, cocoa, baking powder, salt and soda. Add liquid mixture. Stir in zucchini and turn into greased and floured 9 x 13 inch baking pan. Sprinkle the top with chocolate chips and bake at 325 degrees for 40-45 minutes. When cool, frost with icing if desired.