Sweet Potato Casserole With Marshmallows Recipe

moist sweet potatoes, such as ruby or garnet yams (about 6 large potatoes; see note)

unsalted butter (1 stick; 115g)

sprigs fresh thyme and/or sage

(4-inch) knob peeled fresh ginger, finely grated (about 1 tablespoon; 15g)

buttermilk or sour cream

Kosher salt

mini marshmallows (3 ounces; 90g) (see note)