Preheat the oven to 325°F (160°C). Line an 8x8-inch (20x20cm) baking pan with parchment paper or grease thoroughly.
Roughly chop the zucchini and blend until smooth.
In a large mixing bowl, whisk together the zucchini puree and sugar until the sugar begins to dissolve. (This step helps create a shiny top.) Avoid letting the sugar sit too long with the zucchini or it will draw out excess moisture.
Add the oil and vanilla, whisking until fully combined and emulsified.
Whisk until smooth and evenly mixed.
Sift or sprinkle in the flour, baking soda, and salt. Stir just until no streaks of flour remain.
Gently stir in the chopped chocolate or chocolate chips.
Pour the batter into the prepared pan and tap the pan a few times on the counter to remove air bubbles. Bake for 20–23 minutes, or until set on top.
Let the brownies cool completely in the pan before slicing into 16 squares. Cutting while warm will cause them to stick to the knife. For neat slices, wash and dry the knife between cuts.