In a medium mixing bowl of a stand mixer fitted with the whisk attachment (or using a handheld mixer), beat the egg whites with cream of tartar on high speed until frothy, about 3 minutes.

In a small pot, whisk together the sugar and water, bringing it to a simmer over medium heat. Whisk the mixture until it begins to simmer, then stop stirring. If any sugar crystals develop on the side of the pot, use a pastry brush dipped in water to wipe them away. Continue to cook until the syrup reaches 240ºF (116°C) on a thermometer.

With the mixer on high speed, slowly and gently pour the hot sugar syrup in on the side of the bowl. Once the syrup is fully incorporated, continue to whip until the meringue forms stiff peaks, about 8 minutes.

In a medium saucepan over medium heat, bring the milk to a simmer. Add the semi-sweet chocolate, cocoa powder, granulated sugar, and salt. Whisk until the chocolate has completely melted and remove from heat.

Add the meringue to a piping bag or plastic bag. Divide the hot chocolate among mugs. Cut the tip off the bottom corner of the bag and pipe the meringue around the cup.

Use a kitchen torch to quickly and evenly toast the meringue until golden brown all over, without burning it. Top with a bit of graham cracker crumbs and serve immediately.