Sift together the flour and baking soda and set aside. In a mixer with a whip attachment cream the butter until light and fluffy then add the sugars and continue mixing to cream them together. Add the eggs gradually and let them thoroughly mix in. Then add the melted unsweetened chocolate and mix in well. Add the vanilla and 1/3 of the buttermilk and mix, then add 1/2 of the dry ingredients and mix in well. Repeat alternating wet and dry ending with wet.
Place un-waxed paper cups on a sheet pan and fill them 3/4 full. Bake until firm to the touch on the top of the cupcakes, 20 minutes.
Make the frosting: Off the heat, combine the egg whites, cold water, sugar, and cream of tartar in the top of a double boiler. Whisk until smooth. Place over boiling water and whisk constantly or beat with a hand mixer until the frosting is stiff and holds peaks (this will take about 10 to 15 minutes). Remove from the heat and whisk in the vanilla. Let cool, whisking often.
Let cakes cool in the pan, and then frost while still in the cups.