Divide the chicken mixture into four (or eight, depending on whether you are making large or small Kievs) equal portions.
Remove the butter from the fridge and cut into four (or eight) equal portions.
Using wet hands, take a portion of the chicken and flatten it out in the palm of your hand. Place a slice of the butter in the middle of the chicken and use your hands to mould the chicken into an oval shape, ensuring the butter is completely enclosed. Repeat with the remaining portions.
Set up three shallow bowls: one with the flour, one with beaten egg and the other with the crumb coating.
Take each portion, first roll it in the flour, then coat it in the beaten egg, then press it into the breadcrumbs. Make sure that it is well covered in breadcrumbs.
Now place them in the fridge until you are ready to cook them (I recommend chilling them for at least 30 minutes before cooking).