Preheat the oven to 170 degrees Celsius.
For the batter, mix the soy milk and apple cider vinegar together and allow to sit for 5 minutes
Sieve the flour, cacao powder, and baking soda into a large bowl. Add the sugar and mix to combine.
Make sure the butter is soft and at room temperature. Add it to the dry ingredients and rub the vegan butter into the dry ingredients to form a crumb consistency.
Lastly, add the soy milk mixture and the melted chocolate to the dry ingredients and whisk to ensure there are no visible lumps.
Cut 2 circles of parchment paper to fit the bottom of your 2 cake tins and place them into the base of the tins to prevent the cake from sticking. You can also rub some of the vegan butter along the sides of the tins to prevent the cakes from sticking to the sides and make removing them from the tins much easier!
Bake in the oven for 25-30 minutes. After 25 minutes, check they are done by sticking a clean knife into the center of the cakes. If the knife comes out clean, the cakes are ready. If the knife has batter on the sides, allow them to bake further.
To make the frosting, whisk together the ingredients until smooth. (Use an electric beater if you have one). Place in the fridge for 10 minutes to allow the mixture to get slightly thicker.
For the filling simply heat the soy milk and marshmallows in a saucepan until melted. Spread the mixture on top of one of the cakes evenly.
Sandwich the second cake onto the first and coat the entire cake with the chocolate frosting using a spatula.
This cake will store well in the fridge in an airtight container for up to a week. Alternatively, you can cut and freeze it in sections and defrost them before serving.