Here's a secret: dark beer aged in Wild Turkey bourbon casks takes on a quite particular delicious smoothness. Other bourbon casks are available, but none are quite so good as Wild Turkey. Use them twice.
I was gifted some of these casks a few years ago and their first (beer) fill was our 2012 Imperial Russian Stout (long since gone). Their second fill happened in February of this year and this time we used Black Pearl Oyster Stout (vg). The beer takes the flavour on pretty quickly but to get the real depth, and mellow smoothness, time must be added in judicious amounts. This beer reached full term at the beginning of November and was bottled and laid down to condition. I tried it a couple of nights ago, and then again last night, and it's delicious.