Preheat oven to 325° F/160 C. Line a 8 x 8”/20 x 20cm baking pan with parchment or grease very thoroughly.
Roughly chop the zucchini and puree in a blender.
In a large bowl, combine zucchini puree and sugar. Whisk or beat with an electric mixer until sugar starts to dissolve. Dissolving the sugar helps create the shiny top on the brownies. Do not let the sugar sit too long in the zucchini before moving on or all the water will be pulled out of the zucchini creating an unevenly wet batter.
Add in the oil and vanilla extract. Whisk until emulsified.
Add the cocoa powder and whisk until smooth.
Add in the flour, salt, and baking soda. Stir until smooth.
Fold in the chopped chocolate.
Pour into prepared pan and tap the pan several times to force any air bubbles out.
Bake for 20-23 minutes until set on top. Allow to cool completely in the pan before removing and cutting into 16 squares.
Attempting to cut while warm will result in the bars sticking to the knife. For really clean cuts, wash the knife in warm water between slices.
Best the same day but can be kept in a sealed container at room temperature for up to 3 days or freeze for up to 1 month.
Notes
Cocoa Powder: Use regular cocoa powder, not dutch processed. Dutch processed cocoa powder will create a more bitter flavor.Add-ins: Add up to a total of 1 cup/120g of chocolate chunks and/or chopped walnuts, pecans, etc. For extra nuance, add 3-4 tbsp finely chopped candied orange peel or candied ginger. Top with your favorite ganache or chocolate frosting for an even sweeter treat.