Don't use butter, and don't try making it like a 'normal' cake.
This will make a small loaf tin sized cake. If you want to make 2 20cm round sponges to put together, then increase the amounts by 50%
180g s/r flour
20-30g cocoa powder
150g caster sugar
quarter tsp each of baking powder & bicarb of soda.
in a jug/bowl
200ml dairy free milk (soya or oat work well)
20 ml apple cider vinegar
dash vanilla essence
then let this mix sit for about 10 mins ( you can prepare the cake tin/put the oven on, etc)
You'll also need around 70ml vegetable oil
Sieve the cocoa, flour, baking powder & bicarb together, stir in the sugar.
Pour the milk mix in, add the oil, and mix it all together. DO NOT BEAT IT.
If it's very thick, add a little more dairy-free milk.
Pour into your tin and bake.
You can add dairy-free chocolate chips to the mix if you want, or wait and decorate the cake with whatever you have that meets her requirements.