Preheat oven to 350 degrees with the oven rack in the middle position. Line a 12-cup muffin tin with cupcake wrappers.
Combine the flour, baking powder and salt in a medium bowl and set aside.
Cream together the butter and sugar until light colored and fluffy, about 3 minutes.
Add the egg, egg yolks and vanilla and beat at medium speed until thoroughly incorporated.
Add the sour cream and beat until incorporated.
Add the flour mixture and beat until just incorporated.
Spoon the mixture into the muffin cups and bake until a toothpick inserted in the center of a cupcake comes out clean, about 20 minutes.
Remove and cool to room temperature on a rack, about 45 minutes. In the meantime, prepare the frosting of your choice.
In a stand mixer, beat butter until smooth and fluffy, about 3 minutes.
Add chocolate and stir until just combined with a rubber spatula.
Add milk, vanilla and sugar. Beat at medium speed until just combined. (be careful not to overbeat).
Frost the cupcakes with an offset spatula.
Combine the sugar, egg whites, cream of tartar and water in the stainless steel bowl of an electric mixer (i.e. your KitchenAid bowl). Bring a large pan of water to simmer, making sure that the water is a the same height as the egg whites in your stainless bowl. Set the bowl in the pan of simmering water and whisk constantly until the mixture reaches 140 degrees on an instant-read thermometer. Be really careful not to curdle the egg whites– simply remove the bowl from the heat for a bit if it feels like that’s happening. Also, you’ll know when it’s getting close to 140 degrees; the mixture will be getting ultra foamy, like the top of a latte. (A really good latte that is– one of those kinds where it has a leaf-pattern on top, you know what I’m talking about.)
Remove the bowl to your stand mixer and beat on high speed for 3-5 minutes, until the mixture holds glossy, marshmallowy peaks. Remove the meringue to another bowl.
In the standing mixer, beat half the butter and one-third of the meringue until well combined. Continue to add the remaining two-thirds of the meringue a dollop at a time. The mixture may look scarily curdled at this point; adding the remaining butter a tablespoon at a time will smooth things out. (Basically, just keep adding a bit of butter at a time until the curdling corrects itself.)
Using a rubber spatula, stir in the melted chocolate until well-combined.
Frost the cupcakes using an offset spatula.
