Preheat the oven to 180C / 160C fan. Grease and line 2 x 20cm baking tins.
Make the buttermilk by whisking the vinegar into the oat milk. Set aside.
Sieve the flour, sugar, cocoa powder, baking powder and bicarbonate of soda into a large bow. Mix well.
Make a well in the centre and pour in the buttermilk, oil, maple syrup and vanilla
Fold gently until the mixture is just combined.
Divide the batter between both tins. Smooth over with to ensure the mixture is level in both tins.
Bake for 25 minutes until well risen and a skewer inserted in the middle comes away clean.
Remove from the oven and leave to cool in the tins for 5 minutes then turn onto a wire rack and leave to cool completely before icing.
To make the buttercream, remove the thick coconut cream (top part) from the coconut milk and place into a large bowl. Discard the coconut water.
To the coconut cream, add the maple syrup, vanilla and cocoa powder. Whisk until well combined and smooth. Sandwich the cake slices with half the buttercream. Spread over the remaining buttercream on top. Sprinkle over the grated chocolate.
Keep in a cake tin and consume within a day, or store in the fridge and eat within 3 days.