Cut the baby potatoes in half, place in a saucepan and cover with water. Add a large pinch of salt and bring to a boil. Boil for around 15 minutes until potatoes are lovely and tender.
In a mixing cup add sunflower oil, soya milk, salt, apple cider vinegar and mustard powder (if using). Emulsify using a stick blender, making sure it is at the bottom of the cup when you start. If there is any excess oil at the top bring the stick blender up.
Transfer mayonnaise to a large mixing bowl, add finely chopped cornichons, spring onions and dill, along with the wholegrain mustard.
Drain the potatoes, allow to cool slightly.Chuck them into the mayo, stir to combine, season generously with salt + pepper and SERVE.