Chocolate zucchini cake

Cake:
1/2 cup butter
2 cups zucchini
1/2 cup oil
1 1/3 cups sugar
2 eggs
1 teaspoon vanilla
1/2 cup sour cream
2 1/2 cups flour
4 tablespoons cocoa powder
1 teaspoon baking soda 
1/2 teaspoon baking powder
1/2 teaspoon salt
1/2 teaspoon cinnamon

Topping:
1/2 bag chocolate chips
1/4 cup brown sugar

Preheat oven to 350 degrees. Grease a 9- by 13-inch cake pan and set aside. Mix all cake ingredients together. Pour into cake pan and bake for 10 minutes. While that is baking, mix chocolate chips and brown sugar together for the topping. After cake has baked for 10 minutes sprinkle the topping evenly on top and bake for an additional 15-20 minutes.