Pre-heat the oven to 180°C conventional (160°C fan)
Line the two cake tins with greaseproof paper and set aside
In a mixing bowl, beat together the sugar and butter until smooth and creamy
Using a second bowl, split the egg whites into the new bowl (setting it aside for later) and the egg yolks into the bowl containing the sugar and butter mixture
Beat in the egg yolks into the sugar and butter mixture
Using a sieve, sift in the flour, xanthan gum, bicarbonate of soda, baking powder to the bowl containing the eggs, butter and sugar and mix together
Optional: mix in the apple cider vinegar to the plant-based milk to create a vegan 'buttermilk'
Little by little, add in the plant-based milk/buttermilk and then whisk until everything is combined well
Add the crushed Oreos into the cake batter bowl and mix in until combined
Get the bowl containing the egg whites and whisk until stiff peaks form (~8-10 minutes)
Gently fold the egg whites into the rest of the cake mixture using a spatula, ensuring not to mix too much which will cause the whisked in air to escape
Evenly pour the cake batter between the three tins and level out with the spatula if needed
Bake for 32-35 minutes until golden and slightly risen
Remove from the oven after baking and leave in the tins to cool (~30 minutes)
Remove from the tins and place on a wire rack for a further 30 minutes to cool completely
Use a serrated knife to level out the top of the cakes, cutting off any 'dome shaped top' that may have formed during baking