1. To make the sauce, combine all the ingredients in a bowl with a little seasoning.
2. Heat a large non-stick frying pan over a medium heat and brush the cut sides of the buns liberally with melted butter. Toast in the hot pan until really golden and toasted.
3. Turn the heat to high. When the pan is really hot, season the mince and form into four balls, then push two into the frying pan, squashing to flatten. Sprinkle over some salt again and cook for 2 minutes until really crisp, then flip, sprinkle over some salt and repeat. Lay a sheet of cheese onto each and cover with a lid for 30 seconds until melted. Move onto a plate and repeat with the remaining patties and cheese slices.
4. Spread some of the sauce onto the bases of the buns, then top with the cheese patties. Top with lots of pickles, then spread the tops liberally with the sauce and put on top to serve.