Place potatoes in a large pot and add enough cold water to cover potatoes. Add 1 Tbsp salt and bring water to a boil.
Lower heat to a simmer and cook until potatoes are tender when pierced with a fork, about 20 minutes. Drain potatoes using a strainer and set aside.
Return pot to medium-low heat and melt 2 Tbsp butter. Add garlic and cook for 1-2 minutes, until aromatic, but not browning too much.
While garlic is sautéing, combine milk and cream and heat in a small saucepan over low heat or heat in microwave. Heat until hot, but not boiling.
Return potatoes to pot and mash with a potato masher until desired consistency is reached. Take care not to over-mash or else you may get gummy potatoes (I like to leave some chunks, but you can also mash until smooth).
Add remaining butter, hot milk/cream mixture, ½ tsp salt, and ½ tsp pepper and stir to combine.
Serve with chopped scallions or chives and an additional pat of butter, if desired.