Preheat oven to 350 degrees. Lightly oil an 8 1/2" x 4 1/2" loaf pan. Cut a piece of baking parchment to fit inside, with long edges overhanging by 2 or 3 inches and lightly coat the paper with oil, too.
Sift the flour and cocoa into a medium bowl to remove any lumps (a mesh strainer does this job perfectly). Whisk in the baking soda and salt.
In another bowl, whisk the eggs with the sugar, maple syrup and oil until smooth. Pour over the dry ingredients and stir to blend. Add the zucchini and chocolate chunks and stir again just to combine.
Scrape into the prepared pan. Bake 60-70 minutes or until the top feels springy and toothpick inserted in the middle comes out clean. Cool in the pan 15 minutes. Gently lift the bread out of the pan using the parchment as handles (you might want to use a dull knife along the edges of the pan first to release it). Cool on a rack until completely cool.
To top the bread
Put the 1/4 cup chocolate in a microwave-safe bowl and microwave on high for 30-second intervals until almost melted. Stir until smooth. Drizzle the warm chocolate over the bread and sprinkle with pistachios.
Allow the chocolate to set 15 minutes or so, then slice and serve. The bread keeps at room temperature up to 3 days, covered.
This bread freezes well for about a month - wrap the unglazed cake in plastic and place in a ziptop freezer bag (squeeze out the air). Defrost and glaze the cake before serving.
I like to use good-quality chocolate bars chopped into chunks because they usually have a higher cocoa butter content and are not as sweet as chocolate chips. They also tend to melt better and leave larger chocolaty pockets in the baked bread.