Fill a medium saucepan with an inch of water and bring to boil. Combine the sugar, salt, cream of tartar and egg whites in a large mixing bowl (if you have a stand mixer, you'll want to use that bowl), set the bowl above the simmering water.
Heat, whisking constantly, until the sugar dissolves. The mixture will go from a yellowy, granular substance to white and opaque, this takes about 5 minutes. Using a thermometer, it should read 115°F. If you don't have a thermometer, when you pull the whisk up feel the mix with your fingers – if it is no longer grainy then it is ready for the next step.
Put the bowl into your stand mixer with a whisk attachment and beat on medium-high until you get a stiff, glossy meringue – about 8-10 minutes. You'll want to be beating the mixture until the bowl is completely cool.
Add butter one piece at a time, allowing it to incorporate fully, while still at medium-high speed. This should take 8-12 minutes. When you've added all the butter and the mixture looks smooth, slowly pour in the chocolate. Give it one final mix using a rubber spatula to make sure all of the chocolate is incorporated.