Pre-heat oven (180 °C, Fan 160 °C, 350 °F, Gas 4). Grease and line the bottom of two 20cm (8”) round cake tins with baking paper.
Mix the soy milk and apple cider vinegar – it will thicken up. Set it aside for later.
In a large bowl, sift together the dry batter ingredients (flour, cocoa powder, sugar, baking powder, baking soda and salt), then whisk in the liquids (soy milk, olive oil and maple syrup) until a smooth batter forms. Do not beat heavily or for any longer than needed to form a smooth batter.
Divide the batter evenly between the cake tins and bake in the centre of the oven for ~30-35 minutes. Towards the end of this time test by inserting a cocktail stick into the middle of a cake. It should come out clean. If there is wet batter on the stick, return it to the oven and cook for a few minutes more, then test again. Repeat as needed until the stick comes out clean.
When the cakes are cooked, remove them from the oven and gently run a sharp knife around the edge of each cake to separate it from the tin. Place the cakes on a cooling rack and leave until completely cooled before turning the cakes out and discarding the baking paper.