Start by grating the zucchini finely. Once grated, place it in a clean kitchen towel or cheesecloth and squeeze out as much excess moisture as possible to prevent sogginess.
In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, beat the eggs with the granulated sugar and brown sugar until light and fluffy. Add the vegetable oil and vanilla extract, mixing until combined. Fold in the grated zucchini.
Gradually add the dry ingredients into the wet ingredients, stirring gently just until incorporated. Avoid overmixing.
Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat. Drop generous dollops of batter onto the sheet, spacing about 2 inches apart, then flatten each mound slightly with the back of the spoon.
Drop small spoonfuls of chocolate hazelnut spread onto each muffin top and swirl gently with a toothpick or knife. Sprinkle chopped hazelnuts evenly on top.
Bake for 12-15 minutes or until edges are set and a toothpick inserted into the muffin tops (avoiding the swirl) comes out clean or with a few moist crumbs. Cool on the baking sheet for a few minutes before transferring to a wire rack.