Peel sweet potatoes and chop them into 2 inch pieces. Add to large pot of salted water. (I add about 1 tablespoon of salt to the water.) Bring to a boil on the stove top over medium heat. Boil for 15-20 minutes of until the sweet potato pieces are fork tender.
Drain sweet potatoes in colander. Add 6Tbsp butter to the hot pot the sweet potatoes just came from.
Add drained sweet potatoes back to warm pot. Use a potato masher to mash potatoes, mixing in butter. Add in rest of butter, if needed. Mix in garlic, salt & pepper, sour cream and half & half, using the potato masher to mash & mix in all ingredients.
Spoon into serving bowl to serve. Top with an extra pat of butter, chopped parsley or thyme, if desired.