9. Start Meringue
Once the bars are cool, make the meringue. Whisk together the sugar, egg whites, and cream of tartar in the bowl of a stand mixer.
Add about 1 inch of water to a small saucepan. Bring to a simmer over high heat, then reduce the heat to low to maintain the simmer. Place the mixer bowl over the water in the pot, making sure that it is not touching the water. Cook, whisking frequently, until the sugar is dissolved, the mixture is steaming, and the mixture registers 165°F (74°C), about 5 minutes. (If the temperature of the egg mixture stops rising, you can increase the heat under the pot to medium-low.)