Learn French: Le chocolatier prépare du caramel fondant. - The chocolate maker is preparing some melting caramel.

Du is a partitive article used for uncountable quantities, translating to "some" caramel. If it were le caramel, it would mean he is preparing caramel in general, or a specific batch of caramel you already talked about.

What verb does the word fondant come from?

It comes from the verb fondre, which means "to melt". By taking the -ant ending, it becomes a present participle that acts as an adjective here, meaning "melting" or "soft".

Does fondant need to change if the noun it describes is plural or feminine?

Yes! Because it functions as an adjective in this sentence, it must agree with the noun it describes. Caramel is masculine singular, so fondant doesn't need any extra letters. If it described a feminine noun like la crème, it would become fondante.

Could I say est préparant to mean "is preparing"?

No, French doesn't use the verb être (to be) with an -ant verb to show ongoing action. The present tense prépare translates to both "prepares" and "is preparing". To strongly emphasize that the chocolate maker is in the middle of doing it right now, you would use the phrase est en train de préparer.