Preheat oven to 350 degrees. Line a muffin tin with muffin cups and set aside.
Combine flour, sugar, baking soda, cinnamon, nutmeg, and salt in a large bowl.
Whisk together egg, oil, milk, lemon juice, and vanilla.
Make a well in the center of the dry mixture and pour in the wet mixture.
Mix well.
Fold in zucchini and chocolate chips.
Evenly distribute the muffin mixture between the 12 prepared muffin cups.
Prepare streusel topping by mixing together light brown sugar, granulated sugar, flour, cinnamon, and salt. Mix in cubed butter with a fork or pastry cutter until pea-sized balls are formed.
Spoon a heaping teaspoon of streusel over filled muffin tins.
Bake for 22-25 minutes until a toothpick inserted in the center of the muffins comes out clean.