A few hours before serving, make the topping. Combine the egg whites, sugar, vanilla, and salt in the metal bowl of a stand mixer and set over a pan of simmering water. Keep the mixture moving, using a whisk or the whisk attachment, until the sugar is completely melted and it’s hot to the touch (or 160F). Transfer to a stand mixer and beat on high speed until glossy and very stiff, 2 to 3 minutes. Transfer to the top of the lemon bars, smooth out, and use a large serving fork to create a pattern in the meringue.