Making ice cream seems like a good way to get your initial social capital. Like, before you can embark on your hygiene and gem theory programs, and to raise capital for your experiments to create a steam engine, you can bring modern-ish ice cream to whatever town you end up in.
Some sort of sorbets were however "imported" much earlier from the East, recipes being mentioned by Marco Polo, and even earlier there are records of sorbets recipes being brought in Italy by people returning from crusades (probably faloodeh, orginating from Persia/Iran).
It seems like Ruggeri "invented" more moderns sorbets, whilst Buontalenti was the first one to introduce the egg (and thus "proper" ice cream).
Both, directly or indirectly, worked at the Medici court in Florence, but while Ruggeri was actually a cook brought with her by Caterina de Medici in France, and his sorbets seem to have been served in occasion of the marriage, Buontalenti was an architect that worked all life in Tuscany and created the ice cream much later (the marriage of Caterina de' Medici took place in 1533 and, Buontalenti was born in 1531), Buontalenti probably invented ice cream around 1565.
Before Nancy Johnson's machine, the churning was done laboriously by hand.