From the brewery:

This is one of eight releases of barrel aged Imperial Stout. One brew, split into eight different casks and left to develop and absorb the character and flavour embedded in the wood. This one has been resting in a 30 year Pedro Ximénez butt, on cacao nibs, with our Brettanomyces based culture, making an incredibly rich and decadent beer. Heavy with dark chocolate, chocolate liqueurs and warming Christmas spices. These will develop over time but this is excellent to drink right now.