His business started after the acquisition of one of Cailler's factories at Rue des Bosquets, which was therefore first named Peter-Cailler & Cie.[4] One of the main goals of Peter as a chocolatier was the creation of a solid version of the popular chocolate milk beverage, so that it could be more easily transported and consumed.

Moisture also favors mildew, therefore a poor shelf life (earlier attempts had been made with small quantities of fresh milk).

[10] However, it is only in 1887 that the final product was developed, with the help of conching invented by Lindt in 1879, which would refine the chocolate and make it smooth and homogeneous.

[15] During World War II, German secret agents attempted to assassinate British Prime Minister Winston Churchill (who was known to have a sweet tooth) with fake Peter's Chocolate bars.

Underneath a real chocolate exterior were flat slabs of explosive which would have been activated by the breaking of a bar.

[16][17] The attempt on Churchill's life was prevented by the scientist Lord Victor Rothschild, who warned the public to watch out for the fake bars.