Preheat oven to 450 degrees Fahrenheit. Line a muffin tin with 12 paper liners. Lightly grease paper liners with nonstick cooking spray.
In a large mixing bowl, use a hand mixer to cream together 1/4 cup salted butter and 1 1/2 cup brown sugar for about 1 minute, until smooth. Then add in 2 cups grated zucchini and blend for an additional minute.
Beat in 2 large eggs until combined. Then add in 2 teaspoons baking powder, 1/2 teaspoon salt, 1/2 teaspoon ground cinnamon, and 1 teaspoon vanilla extract. Mix briefly until just combined.
Add in 1 cup of flour and mix until just combined. Use a rubber spatula to scrape the bottom and sides of the bowl. Then add in the remaining 1 cup flour until just combined. Fold in mini chocolate chips or chopped nuts, if using.
Divide the batter equally amongst the 12 muffins, filling almost to the top of the muffin tin.
Bake in the preheated oven 450℉ oven for 5 minutes. Reduce oven temperature to 350℉ and continue baking another 15-17 minutes, until a toothpick inserted into the center comes out clean.
Let muffins cool for 5 minutes in the pan before removing and transferring to a wire rack to cool completely.