Preheat the oven to 180°C (350°F).
Mix the flax eggs and set aside.
Mix the plant-based milk and apple cider vinegar, set aside.
In a large mixing bowl, combine the flour, sugar, baking powder and salt.
Add the milk mixture, flax eggs, orange zest & juice and the oil and mix gently with a large wooden spoon. Add more plant-based milk, if the batter is too thick. Be careful not to overmix!
Fold in the marmalade and the chopped chestnuts.
Spoon the batter into silicon muffin cups or a muffin tray lined with paper cases, leaving enough space for the muffins to rise.
Bake for 20–25 mins until an inserted toothpick comes out clean.
Transfer them to a wire rack and let them cool.
Sprinkle with icing sugar or brush them with warmed-up marmalade glaze.