Wash the potatoes but leave the skins in place, cut roughly into 1” cubes and put all in a pan cover ¾ to 7/8 of the way with cold water and add 1 tablespoon of the sea salt, use 1 cup of the beef stock to finish covering the potatoes. Cover the pot and bring to a boil, once at a boil remove the lid and reduce the heat to low. Cook until the potatoes are easily mashed with tongs.
While the potatoes are cooking take a large sauté pan and melt the ½ cup of butter on a medium high heat until just slightly brown, remove all the butter to a heat proof measuring device and set aside. Add the cabbage to the pan and half of the brown butter to the pan to help sauté the cabbage. Once the cabbage has started to soften add the mushrooms to the pan and cook until the cabbage is well softened and add the remaining salt, all the pepper and two shots of the whiskey to the pan and toss the cabbage and mushroom mixture until well coated. Remove to a large bowl and set aside.
Take 1 shot of whiskey and drink it.
Deglaze the pan with the remaining whiskey, add the remaining cup of beef stock, the brown butter to the pan, and the remaining tablespoon of pepper. Reduce to a thin gravy over a low heat.
Drain the potatoes and put them back into the large pot, the drier the potatoes the better. Add the cabbage & mushroom mixture and the milk and mash to the desired consistency.
Serve in a bowl and make a crater to add the gravy to the middle.