Start by preheating the oven to 400°. Using a cheese grater, grate ½ stick of butter. The long, thin strands of butter guarantee a biscuit that’s flaky AF. To keep the butter cold, which also promises a well-layered biscuit, pop it into the freezer once it’s grated. While you’ve got the grater out, shred some cheese—you can use any kind of hard cheese you like, but I’m partial to a super sharp cheddar—until you’ve got ¾ cup cheese, plus a little extra for topping the biscuits.
In a large bowl, whisk together 2⅓ cups all-purpose flour, 2 tsp. salt, and 1 Tbsp. baking powder. Gather up whatever herbs you’ve got and finely chopped enough so that you’ve got 2 Tbsp. I like a combo of rosemary, sage, and thyme. Chives, parsley, or dill all work well too.
Add the chilled, grated butter to the dry ingredients and toss lightly just until the butter is coated in flour. Then, add 1 cup leftover mashed potatoes and continue tossing lightly until the potatoes have broken down into pea-sized pieces. There’s no need to worry so much about whether your mashed potatoes are super fatty or plain Jane—I’ve made this recipe with both, and both work great.
Using your hands, make a well in the middle of the ingredients. Fill the well with ¾ cup buttermilk. Slowly mix the ingredients together until a dough forms. Turn the dough out onto a floured surface and pat down into a 10-inch circle.
To cut the biscuits, grab an empty soup and remove the lid from the bottom so you have a hollow cylinder. Using the empty can, cut 10–12 biscuits. (If you’d rather use a 3-inch biscuit cutter, that works too.) Once cut, place them into a 10-inch cast-iron pan.
Brush the tops with buttermilk and top with more cheese, a sprinkle of herbs and a pinch of freshly cracked black pepper. Bake the biscuits for 20–25 minutes or until they’ve doubled in size and are golden brown on top.
Enjoy the biscuits warm, with a slather of butter, served alongside some eggs or, for the ultimate leftover treat, break open a biscuit to build a slider-sized, but also superior, turkey sandwich. Once you’ve tried ‘em, I can all but guarantee that next Thanksgiving you’ll peel an extra potato or two with the next day’s biscuits in mind.