The frosting may be refrigerated in an airtight container for up to 3 days. Bring to room temperature and re-whip with an electric mixer fitted with the paddle attachment before using.
In the heatproof bowl of an electric mixer, combine egg whites, sugar and salt over a pot of simmering water. Whisking constantly, cook until sugar and salt dissolve and mixture is warm to touch (about 160 degrees). Using the whisk attachment, beat mixture on low speed and gradually increase to high, beating until mixture is stiff and glossy, about 10 minutes. With mixer on low, add butter a few tablespoons at a time, then whisk in vanilla. Using paddle attachment, beat buttercream until air bubbles are gone and mixture is smooth. Gently fold in crushed cookies. Use immediately.