First, cook the macaroni in a large pan of salted water for 6 mins, or 1-2 mins less than it says on the packet. Drain and set aside.
Meanwhile, make the sauce. Melt the butter in a large saucepan and add the garlic and mustard powder, stirring for 1 min. Tip in 1 1/2 tbsp of plain flour and stir for 1 min. Gradually pour in the milk, whisking until the sauce is lump-free, then simmer for 5 mins, whisking constantly until it thickens and becomes glossy.
Add the cheeses, truffle oil and seasoning, then stir through the macaroni. Spread over a large baking sheet and leave to cool. Cover with cling film and put in the fridge to chill for at least 1 hr, or overnight.
Weigh the chilled macaroni cheese into 30g portions, roll into 15-20 balls and put back on the baking sheet.
Tip the flour onto a plate and season with salt. Pour the beaten eggs into a bowl and tip the breadcrumbs into a dish. Coat each ball in the flour, tapping off any excess, then dip in the egg and roll in the breadcrumbs. Dip the coated balls back in the egg mix and roll in the breadcrumbs again for a super-crispy coating. If making ahead, they can now be left in the fridge overnight or frozen for up to 3 months (defrost before cooking).
Heat the oil in a deep-fat fryer or large saucepan until it reaches 160C on a temperature probe, or a piece of bread browns in 40 secs. Heat the oven to low. Fry the macancini in batches for 2-3 mins until golden and piping hot in the centre. Drain on kitchen paper and transfer to the oven to keep warm while you fry the rest. Serve immediately with a good grating of Parmesan, and tomato sauce for dipping.