In a 4- to 5-quart pan over high heat, bring 2 quarts water and 1 teaspoon salt to a boil. Add potatoes. Cover and return to a boil, then reduce heat to medium and simmer until potatoes mash easily, 8 to 10 minutes. Drain potatoes and return to pan.
Meanwhile, in 1- to 1 1/2-quart pan over low heat, warm 3/4 cup milk until steaming (do not boil), about 5 minutes; or heat in a microwave-safe container in a microwave oven on full power (100%), stirring occasionally, about 1 minute.
Add 2 tablespoons butter and 1/2 cup of the hot milk to potatoes and mash with a potato masher or an electric mixer on medium speed until mixture is as lumpy or smooth as you like, gradually beating in up to 1/4 cup additional milk and more butter, if desired, until potatoes reach desired consistency. Add salt and pepper to taste.