Make Ahead and Storage: The cookies can be stored airtight for up to 1 week. Why It Works: This recipe is all about ratios. The right amount of butter for flavor, flour for structure, and sugar for moisture and tenderness, as well as for sweetness, yield a perfectly crumbly cookie. I like to brush the shortbread with egg wash to five it a lightly golden, crackly look. If you leave it plain, the cookies will have a pale, smooth surface. Both versions are pretty and delicious, so do as you please! Pro Tip: Now is the time to use the fancy butter if you can--with so few ingredients, the rich flavor of higher-fat Irish or European butter makes for truly extraordinary shortbread. How to Keep Your Brown Butter Soft: Until I discovered this trick, when I dug into the brown sugar jar, I often encountered a hard, unscoopable mass. But guess what: Throw one or two marshmallows into the container, and they'll prevent the sugar from drying out. And since marshmallows are shelf stable, you don't have to worry about replacing them.