Deep Dark Chocolate Sour Cream Zucchini Cake

Ingredients

• 1/2 cup butter, at room temperature
• 1/4 cup canola or other mild vegetable oil
• 1 3/4 cup sugar
• 2 large eggs
• 1 tsp. vanilla
• 2 1/4 cups all purpose flour
• 3/4 cup cocoa
• 1 tsp. baking powder
• 1 tsp. baking soda
• 1/2 tsp. salt
• 1 cup sour cream or plain yogurt
• 2 cups grated unpeeled zucchini (about 2 smallish, or 1 large)
• 3/4 cup chopped walnuts or chocolate chips, or both (optional)