Combine ground flaxseed with 1,5 tbsp boiling hot water (as written in the original recipe / adjust accordingly if you changed the portion size). Stir and set aside.
In a separate cup, combine plant milk and vinegar to create vegan sour milk. Stir and set aside.
Use a fork to mix melted dairy-free butter and sugar until smooth.
Add the butter mixture, flax mixture and vanilla to your sour milk. Mix well.
Sift all your remaining dry ingredients (cocoa, flour, baking powder).
Mix, then slowly pour your wet ingredients over your dry ingredients whilst stirring the mixture.
Mix until no more flour specks can be seen but do not overmix.
Line your cupcake tin and place one Oreo cookie at the bottom of each cupcake mould.
Pour your batter on top of the cookie to fill the mould about ¾ full.
Bake at 180°C / 356°F (fan oven) for 20-25 minutes.
In the meantime, prepare your icing (instructions below).
Once baked, leave your cupcakes to cool on a cooling rack.
Once cool, fill a piping bag with your icing and start icing. Decorate with an extra Oreo on top. Sprinkle more Oreo pieces on top if desired.