White chocolate was actually an accidental byproduct of which product?

In the 1930s, Nestlé was working with Roche to develop 'Nestrovit,' a vitamin-enriched milk supplement for children. To turn the nutrient-heavy milk powder into a solid tablet, they added cocoa butter. This 'fortified milk' accidentally resulted in the creamy, non-dark treat we now know as white chocolate, eventually launched as Milkybar.