Combine plant milk and vinegar to create vegan sour milk. Stir and set aside.
Mix melted dairy-free butter and sugar until smooth using a fork/spoon.
Add the butter mixture and vanilla to your sour milk. Mix well.
Sift all your remaining dry ingredients (cocoa, flour, baking soda).
Mix, then slowly pour your wet ingredients over your dry ingredients whilst stirring the mixture.
Mix until no more flour specks can be seen but do not overmix.
Add your peanut butter on top of the mixture and stir very briefly.
Pour your cupcake batter into your lined cupcake tin filling them almost all the way to the top (about ⅘ full).
Add a tsp of peanut butter to each cupcake. Create a swirl using a skewer, chopstick or a thin knife.
Bake at 180°C / 356°F (fan oven) for 20 minutes,
In the meantime, prepare your icing (instructions below).
Once baked, leave your cupcakes to cool on a cooling rack.
Once cool, fill a piping bag with your icing and start icing. Drizzle with sauce if desired.