Dark Chocolate Zucchini Brownies

Brownies: Preheat oven to 350 degrees.  Grease a 9 x 13 baking pan with nonstick cooking spray.  In a blender, puree zucchini, oil, yogurt, sugar and vanilla.  Set aside.  In a large bowl, whisk together flour, cocoa, baking powder and salt.  Add wet ingredients to dry ingredients and stir until thoroughly moistened.  Pour batter into greased pan, spread evenly and bake for 20-25 minutes, or until a toothpick comes out clean.  Remove from oven and let cool for 20 minutes.  Frost while still warm. Frosting: Combine chocolate chips, evaporated milk, butter and vanilla in a microwave safe bowl.  Microwave for 30-40 seconds.  Remove bowl from the microwave and whisk until chocolate chips are melted and your frosting is smooth.  Add powdered sugar and whisk together until smooth.  Spread your frosting evenly over your warm brownies.  Sprinkle with chopped nuts, if desired.