To make the butter, add all the ingredients into a bowl and season well with pepper and a little salt. Mix until completely combined then slide onto a sheet of cling film and roll into a cylinder. Place in the fridge to set until hard. This can be made 3 days in advance, or can be kept frozen for u to 3 months.
Take one chicken breast at a time, lay smooth side facing down and using a knife cut off the mini fillet. Make a slit down the middle of the fillet about half way into it to form a pocket;making sure not to cut the full way through the meat. Place a piece of cling film over the chicken breast and, using a rolling pin, flatten it out just a little. On another part of the board flatten out the mini fillet slightly.
Divide the butter into four parts and flatten into into flattish discs. Push a disc of butter into each of the pockets Cover each pocket with the mini fillet of chicken and fold the sides of the breast over it. Leave aside.
Add the flour, eggs and breadcrumbs into different shallow bowls. Coat each breast completely in flour, then egg, then breadcrumbs, then dip back into the egg and lastly once again in the breadcrumbs. Leave to one side. The chicken can be prepared the day before and left in the fridge or frozen for up to 1 month - defrost completely before cooking.
To cook, preheat oven to fan 180°C/gas 6. Heat a generous splash of oil in a large frying pan and, once hot, reduce the heat to medium. Fry the Kievs for 2-3 minutes on each side until dark golden. Lift them onto kitchen paper to remove any excess oil then place on a roasting tin. Cook in the oven for 20-25 minutes or until the Kievs. Serve immediately with the lemon wedges and watercress, plus a retro salad of your choice!