Peel, quarter and boil the 300g potatoes until fork tender then drain. Add into a large mixing bowl. Mash until smooth and season with a little salt and black pepper. Grate the potatoes and add into the bowl.
Finely chop the spring onions and savoy cabbage if using. Add to the bowl along with the buttermilk, flour, baking powder , garlic powder and wholegrain mustard. Stir together until combined. You want this to be a very sticky texture, so add the extra 50g of buttermilk if needed.
Add a very generous tablespoon of butter into a non stick frying pan and form patties from the mixture using your hands. Gently press down to flatten into pucks into the frying pan and fry for ⅘ minutes on a low/medium heat.
Turn the boxty and fry for 3 mins, and add the cheese. Serve!
