Whisky aged in Tawny casks is aged in barrels that were originally used to store fortified wines such as port or sherry.
The wood from these barrels imparts a rich, sweet and fruity flavour to the whisky. The specific flavour profile can vary depending on the type of fortifying wine that was previously stored in the barrel, but common flavours include dried fruits, nuts, honey, and vanilla.
The barrel provides a mellowing effect over time, making the whisky smoother and more complex. The wood also imparts tannins, which can add astringency and a slight bitterness to the finished whisky.