How to Make It
Heat oven to 350° F. Brush an 8½-by-4½-inch loaf pan with oil and dust with flour, tapping out the excess.
Stir together the chocolate and 2 teaspoons of the canola oil. Set aside to cool to room temperature.
Whisk together the flour, cinnamon, baking powder, baking soda, and ¾ teaspoon salt in a medium bowl; set aside.
In a separate bowl, whisk together the remaining ½ cup canola oil, sugar, eggs, and vanilla. Stir in the zucchini. Add the wet ingredients to the dry ingredients and whisk vigorously to combine, about 1 minute.
Transfer half the batter to the prepared pan. Pour half the chocolate into the pan. Repeat with the remaining batter and chocolate. Using a toothpick, swirl the chocolate and batter together to create a marbled pattern. Make sure to incorporate the batter at the bottom.
Bake until a toothpick inserted in the center comes out clean, 60 to 70 minutes. Cool in the pan for 15 minutes. Transfer to a wire rack and let cool completely.