Simply cut open the panini bread and butter the inside. You don't need to butter the outside of the bread with a panini maker, but I know some people still do. My eldest doesn't like butter, so his is made without any butter at all.
Dice the chicken breasts and cook them fully, whether in a frying pan, in the oven, or steamed. This is also a good recipe for left over chicken from the previous day.
Mix the cooked chicken breast with the mayonnaise and fill the opened panini. Add salt and pepper if you wish. Top it with the lettuce and close as you would a sandwich.
Pop into the panini maker, close the lid and leave for approximately 5 - 10 minutes.