Preheat the oven to 350°F. Line two cupcake or muffin pans with papers, and spray the papers with non-stick vegetable oil spray, if desired., Prepare the cake batter from our Favorite Fudge Birthday Cake recipe., Divide the batter evenly among the cupcake cups; you'll pour a scant 1/4 cup batter (about 58g) into each cup. A muffin scoop works well here., Bake the cupcakes for about 25 minutes, until a cake tester or toothpick inserted into the center of one of the cakes in the middle of the pan comes out clean., Remove the cupcakes from the oven. In about 5 minutes, or as soon as you can handle them, remove them from the pan, and place them on a rack to cool., When the cupcakes are cool, make the frosting. Place the 2 egg whites in a large mixing bowl or the bowl of your stand mixer. Beat until the egg whites are foamy and thick; they should mound in the bowl, without holding a peak. Set them aside while you prepare the sugar syrup., Combine the sugar, cream of tartar, water, and salt in a saucepan. Bring to a boil, stirring frequently; the sugar should be dissolved. If the sugar hasn't dissolved, cook and stir a bit more, until it has. Once the sugar has dissolved, boil the syrup, undisturbed, for 2 minutes, or until the syrup registers 240°F on a candy thermometer or instant-read thermometer., Begin to beat the egg whites, and immediately pour the boiling sugar syrup into the egg whites in a slow stream, beating all the while. As you beat, the mixture will thicken., Once all the syrup is added, stir in the vanilla, and continue to beat until the frosting is thick and will hold a peak., Spoon the hot frosting atop the cooled cupcakes, swirling it decoratively., Decorate while warm with sprinkles, mini chocolate chips, sparkling sugar, or the decorations of your choice.