Preheat oven to 350°F (175°C) and line a 12-cup muffin tin.
Grate zucchini and carrots. Squeeze moisture from zucchini.
In a bowl, beat eggs, then mix in vanilla and brown sugar.
Stir in grated zucchini and carrots.
In another bowl, whisk flour, baking powder, baking soda, and cinnamon.
Combine dry ingredients with wet, stirring gently. Fold in optional mix-ins.
Divide batter among muffin cups, filling two-thirds full.
Bake 18-22 minutes, until toothpick comes out clean.
Cool in tin briefly, then transfer to wire rack to cool completely.
