Theoretically, anything *can* be served warmed, but with beverages, specifically alcoholic ones, there is a big difference between serving it warm (unchilled) or warmed, (heated). Cognac does have an ettiquete attached to it that you should hold the glass for a little to warm it slightly above room temparature, but it should not be heated. Higher end alcholols, and a number of cognacs are commonly viewed to be in that family, if still in the middle of the spectrum quality-wise when it comes to whiskey (or whisky depending on which you are referring to) are mostly served "neat", which is simply the alcohol in a glass. Chilling alcohol usually dilutes the taste of a higher quality product at the benefit of making it easier to drink and it thus not commonly done. Cognac can be chilled, It is most commonly served warm (room temparature), but the "warming" from the heat your hand gives the glass is supposed to enhance the flavor to its full potential.
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