Make the meringue: Whisk the egg whites, sugar, and cream of tartar (if using) in the bowl of a stand mixer. Make sure the bowl and whisk are completely clean; any residual oil will prevent stiff peaks. Set the bowl over a 2QT Stainless Clad Saucepan with 1 inch of water, ensuring the bottom of the bowl doesn’t touch the water. Heat over medium, whisking constantly, until the mixture reaches 160F on an instant-read thermometer. Remove from the heat.

Transfer the bowl to the mixer fitted with a whisk attachment and whip until the meringue reaches stiff peaks. Add the vanilla and salt and mix to combine. Let cool as you make the hot chocolate.

Make the hot chocolate: Add the milk and brown sugar to a 3QT Stainless Clad Saucier and bring to a gentle simmer over medium heat, whisking often. Reduce the heat to medium-low and add the chopped chocolate. Whisk until the chocolate is melted and fully incorporated. By the time this happens, the color should be a rich brown. Use a silicone spatula to scrape the bottom of the Saucier to make sure none of the chocolate settles. Stir in the salt and vanilla.

Divide the hot chocolate among mugs. Load the meringue into a piping bag and pipe around the edge of each mug. Torch the meringue all over to toast it. Top each mug with a sprinkle of flaky salt (if using) and enjoy!