What links the words croissant and cereal, apart from them often being eaten for breakfast?

A croissant is a flaky roll or pastry in a form of a crescent. Although they’re associated with France, they’re based on the Viennese kipferl, a crescent-shaped pastry dating back to at least the 13th century. They became popular in France after August Zang, an Austrian artillery officer, set up a Viennese bakery in Paris in 1839 which sold Viennese pastries, including the kipferl. Other bakers copied this and created the croissant [source].

The word croissant comes from French, and is the present participle of the verb croître (to increase, grow), from the Old French creistre (to grow), from the Latin crēscō (I grow, from the Proto-Indo-European root *ḱer- (to grow, become bigger) [source].

The word cereal comes from the same roots, via the French céréale (cereal), the Latin Cerealis (of or relating to Ceres), from Ceres (Roman goddess of agriculture), from the Proto-Indo-European root *ḱer- (to grow, become bigger) [source].

Other words from the same roots include create, creature, crecent, crew, increase and sincere [source].

Sincere? That comes from the Middle French sincere (sincere), from the Latin sincerus (genuine), from the Proto-Indo-European roots *sem- (together) and *ḱer- (to grow). So you could say that being sincere involves growing together, perhaps. [source].

It doesn’t come from the Latin sine (without) and cera (wax). That is a popular folk etymology. One story is that dishonest Roman and Greek sculptors covered flaws in their work with wax. A sculpture “without wax” was therefore an honest or sincere one. Another story is that Greek sculptors made fake marble statues out of wax to offer as tribute to their Roman conquerors [source].