
Cognac is a French grape brandy from around the town of Cognac, made under strict AOC rules. It is distilled twice in copper pot stills and aged in oak.
Swaps straight in and is fuller and earthier. In a drink with plenty of other flavors the difference is small.
Spanish or Armenian included, goes in at the same amount. It keeps the oak and the amber color but is usually rougher than Cognac.
Unaged or non-grape brandies (pisco, calvados)
They change the drink.
"Cognac" on post-Soviet shelves
Armenian, Dagestani and Russian "cognac" are local grape brandies. In the EU the word is reserved for the French AOC spirit, so a cheap "5-star cognac" and a true Cognac are different things.
Rye whiskey, Cognac, Rosso vermouth