Place the apples in a 9x13-inch baking dish. Scatter the the brown sugar and chunks of butter evenly over top. Cover tightly with aluminum foil and bake for 45-50 minutes, tossing once halfway through, until the apples are very tender.
Mash the apples with a potato masher or fork. Taste; if the applesauce is too tart, add more brown sugar little by little until the flavours are balanced. (Note that no two batches will ever be the same; the flavour will depend on the tartness of the apples you use.)