Butcher blocks are either not finished at all, or are finished with oil.
There exist special 'Salad Bowl' oils that work well for butcher blocks, in that they are totally non-toxic, but they can be difficult to locate.
Probably the easiest finish is mineral oil from the drug store. The key is to make sure that the butcher block is at least at room temperature or a little warmer, and warm up the mineral oil by putting the bottle in hot water, like a warming up a baby bottle. The warm oil put on warm wood assures good penetration even in maple and bamboo, woods that have very tight grains.
Why mineral oil rather than vegetable oil? Because the mineral oil is non-toxic and it will not go rancid.