Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line two 8" cake tins (that are at least 2" deep)
In a mixing bowl, stir together the caster sugar, self raising flour, cocoa powder, bicarbonate of soda and baking powder
Add the orange zest and mix in it
In another mixing bowl or large jug, whisk together the vegetable oil, dairy free milk, orange extract and cider vinegar
Make a well in the centre of the flour mixture, and pour in the oil and milk mixture, whisk them together gently with a hand whisk until smooth
Divide the mixture between the tins, use scales for accuracy if you like
Bake them for 35-40 minutes or until a skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or remove from the tin and place on cooling racks
To make the buttercream, mix the dairy free block butter on it's own for a few minutes. Then add the icing sugar, cocoa powder, orange extract and dairy free milk, and mix until smooth. For best results, use a stand mixer with the paddle attachment, or a handheld electric mixer. If the buttercream is too stiff, you can add a little more dairy free milk
If the cakes have domed on top, you can level them off with a cake leveller or a knife
Put one of the sponges on your plate or cake stand and pipe or spread some of the buttercream onto it
Add the other sponge on top and pipe or spread the remaining buttercream on top
Decorate with the choccy orange pieces and orange zest
Leftovers will keep in an airtight container in a cool place for 2-3 days
Notes
I used oat milk for the cake, but you can use any plant milk that you prefer.
For the best flavour, make sure to use a good quality orange extract and not an essence.
You can use a different type of oil besides vegetable oil. I'd recommend one that doesn't have a strong flavour, canola and sunflower oil would both work well.
You do need to use the cider vinegar - it reacts with the raising agents to help the cake rise. Don't worry it won't make the cake taste of vinegar! You can also use white wine vinegar.
I've found that the best dairy free butter for vegan buttercream is Flora Plant. Stork Baking Block is also very good. Using a block dairy free butter is much better for the buttercream. A margarine or spread style dairy free butter will make the buttercream too loose.
I used Buttermilk Choccy Orange Segments for the decoration, they are dairy free, gluten free, palm oil free and vegan. Another option is the Nomo Chocolate Orange Crunch Bar or the Galaxy Smooth Orange Chocolate Bar, you could add a chunks of them to the top of the cake. You could also make your own chocolate orange segments like I did for my Vegan Chocolate Orange Cupcakes.
Although I provide cup measurements, I highly recommend weighing your ingredients out using digital kitchen scales. It is the most accurate way to measure ingredients and will ensure the best results. Digital scales are very low cost and can be purchased for around £12 ($16.50) .
For teaspoon (tsp) and tablespoon (tbsp) measurements, please use measuring spoons and not the type of spoons you eat with. Again this will ensure accuracy and provide the best results.