Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a small bowl, mix the almond milk and apple cider vinegar. Let it sit for a few minutes to curdle.
In a large bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, cinnamon, and salt.
Add the vegetable oil and vanilla extract to the almond milk mixture.
Pour the wet ingredients into the dry ingredients and mix until just combined. Fold in the chopped walnuts if using.
Divide the batter evenly among the muffin cups.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for a few minutes, then transfer to a wire rack to cool completely.