Wash & dry breasts then remove skin,.
Place smooth side on cutting board, & pound to flatten to about 1/4" thickness.
Combine 1/2-lb of butter with yolk of egg & divide into 4 equal parts.
Roll into sausage shapes approximately 3 1/2" in length.
Cover each with wax-paper & chill until they very are hard.
Wrap breasts around each piece of butter thus creating 4 packets.
Dip each packet in flour, shake off excess then pat flat in your hand.
Dip in beaten eggs & roll in breadcrumbs.
Refrigerate for a 2-4 hrs.
Fry in a 1/2 lb of butter until golden brown.

*NOTE:
In Ukraine, Chicken Kyiv is served with kasha & peas in pastry shells.
If you reheat or try to warm this dish for any period of time because the butter will melt and leak out.