Preheat the oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease it lightly.
Mix dry ingredients: In a large bowl, whisk together the flour, baking powder, baking soda, sugar, salt, and cinnamon.
Combine wet ingredients: In a separate bowl, mix together the plant-based milk, oil, vanilla extract, apple cider vinegar (or lemon juice), and flax egg or applesauce.
Add wet ingredients to dry: Pour the wet ingredients into the dry ingredients and gently mix until just combined. Be careful not to overmix.
Fold in add-ins: Add berries, chocolate chips, or nuts (if using) and gently fold them into the batter.
Fill the muffin tin: Spoon the batter into the muffin cups, filling each about 2/3 full.
Bake: Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are golden.
Cool: Let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.