Melt the butter in a skillet over low heat and sauté the celery, onion, and garlic until vegetables are soft, about 7 minutes to 10 minutes.
Add the half and half and bouillon and simmer the mixture slowly, uncovered, over low heat for 20 minutes.
Whir the simmered mixture in a a blender or food processor until well pureed.
While the celery mixture is simmering, cut the potatoes into 2-inch chunks (peeled or not, it's up to you) then put them in a stockpot with lightly salted water to cover and cook until the potatoes are tender; drain well.
Combine the celery puree with the cooked potatoes in the stockpot and, over medium heat, mash with a hand masher, adding buttermilk to achieve desired consistency for the mashed potatoes.
Season to taste with salt and pepper.
Makes about 6 servings.
Note: I do not recommend using electric mixers to make mashed potatoes because they can break down the starches in the potatoes too quickly and cause them to become"gluey"- use a hand masher, it's worth the extra bit of effort.