Cubes from genuine, recently-used whisky barrels (American oak). Use these cubes just as you would employ classic oak chips. Add them to beer or liqueur for a few weeks during the fermentation/maturation, taste regularly to check how the flavour is evolving and then savour the final product.
Tips for use:
Sanitise the oak cubes before use by burning ¼ sulphur wick in a sealed receptacle containing the cubes and letting them sit for 15 minutes.