Yes, you can! The flavor of the mashed potatoes will be slightly altered, as the buttermilk adds a slight tang to the mashed potatoes. If you simply don't have buttermilk, you can add 1 tbsp of vinegar or lemon juice to a measuring cup, then add enough milk to make 1 cup. Let that sit for about 10 minutes and you've made your own homemade buttermilk. Alternatively, you can achieve that tangy flavor with the addition of sour cream or Greek yogurt.