Make the cupcakes: Preheat oven to 300F. Line two 12-cup muffin tins with 20 wrappers. Set aside.
In a double-boiler over medium heat, melt butter and chocolate together until smooth. Remove from heat and cool slightly. Once cooled, whisk egg yolks and sugar into chocolate and set aside.
In a separate large bowl, beat egg whites on high speed until egg whites reach stiff peaks. Very gently fold egg whites into chocolate mixture, gently folding a few times until batter is just combined.
Fill prepared muffin cups 3/4 full of batter. Bake cupcakes at 350F 20-25 minutes or until centers are just set. Cool cupcakes on a wire cooling rack (centers are going to sink a little bit)
Make the glaze: In a small saucepan over medium heat, melt chocolate and milk until smooth. Remove from heat and whisk in salt and vanilla.
To serve cupcakes, spoon glaze on top of cupcakes. Enjoy!