The batter is made rich and delicious with the addition of sour cream, but the important thing here is the frosting. If you have never made a seven-minute frosting, or if it has been awhile as it had been for me, go now to start separating some egg whites. I don’t know how I let so much time pass since I’d last made this kind of frosting, but it is a simply wonderful thing. Egg whites are beaten in a bowl with water, sugar, and cream of tartar. The bowl is set over a pot of barely simmering water, and you continue to beat for seven minutes. A glossy, thick, luscious frosting develops before your eyes. After seven minutes, the bowl is removed from the pan of simmering water, vanilla is added, and you mix for another couple of minutes. What you get is the best possible incarnation of marshmallow fluff.
Next, you get to apply this fluffery to the cupcakes, and that is almost as much fun as eating it. You swoop and swirl and let the frosting take whatever crazy shape it can. Because of the lightness of the frosting, this is one that is best applied liberally. My spinning instructor was surprised and delighted by the cupcakes, and he offered to share with everyone—after class of course. Luckily, everyone felt like they deserved a cupcake or two after that class.