Preheat your oven to 350°F (175°C). Grease your baking pan with a little butter or line it with parchment paper for easy removal later.
In a large mixing bowl, combine the whole wheat pastry flour, unsweetened cocoa powder, baking powder, and salt. Whisk them together until well blended.
In another bowl, mix the melted unsalted butter, brown sugar, egg, sour cream or Greek yogurt, milk, and vanilla extract. Whisk until smooth and creamy.
Grate your zucchini using a box grater or food processor. Make sure to squeeze out any excess moisture, as this will prevent the cake from becoming too soggy.
Add the grated zucchini to the wet ingredients, stirring to combine. Gradually fold the dry mixture into the wet ingredients until just combined. Be careful not to overmix.
Fold in the dark chocolate chunks/chips, ensuring they are evenly distributed throughout the batter.
Pour the batter into your prepared baking pan and smooth the top. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.
While the cake is baking, make the candied pistachios by combining the brown sugar and hot water in a small saucepan over medium heat. Stir until the sugar dissolves, then add the roasted pistachios and cook for 2-3 minutes until the nuts are coated and the mixture thickens slightly. Remove from heat and let cool.
Once the cake is baked, remove it from the oven and allow it to cool in the pan for about 10 minutes. Transfer it to a wire rack to cool completely. Melt the remaining dark chocolate chips/chunks with coconut oil in a microwave-safe bowl, stirring every 30 seconds until smooth. Drizzle the chocolate over the cooled cake and top with candied pistachios for garnish.
