1) Place a rack in the middle of the oven and preheat to 190C/Gas 5. Line the muffin pan with paper cupcake liners.
2) In a standing mixer fitted with the paddle attachment, combine the butter and sugar and mix on low speed until just incorporated. Raise the speed to high and mix until light and fluffy, about 10 minutes. (Occasionally turn the mixer off, and scrape the sides of the bowl down with a rubber spatula.)
3) Meanwhile, in a medium bowl, whisk together the flour, baking powder and salt. Set aside. In a small bowl, whisk together the milk and vanilla, and also set aside.
4) Add the egg yolks to the creamed butter one at time, waiting for each one to be fully incorporated before adding the next.
5) Reduce the speed of the mixer to low. Alternately, add the flour mixture in three additions and the milk in two additions, waiting for each to be fully incorporated before adding the next (scrape the bowl down occasionally). Raise the speed to medium and mix briefly until a smooth batter has formed. Transfer the batter to a large bowl.
6) Thoroughly clean the bowl of the mixer and put the egg whites inside. Whip the egg whites on high speed, using the whisk attachment, until stiff peaks form.
7) Working in three batches, using a rubber spatula, fold the egg whites into the batter, until just incorporated. Divide the batter evenly among the cups in the muffin pan. Bake, rotating the pan once, until golden brown and a toothpick inserted in the centre of the cakes comes out clean, about 30 minutes. Remove the cakes from the oven and cool completely.
Using a palette knife or rubber spatula, spread some of the icing all over the tops of the cakes and decorate with the sprinkles.
Cream cheese icing:
1) In a standing mixer fitted with the paddle attachment, or with a handheld electric mixer in a large bowl, mix the cream cheese and sugar on low speed until incorporated. Increase the speed to high, and mix until light and fluffy, about 5 minutes. (Occasionally turn the mixer off, and scrape the down the sides of the bowl with a rubber spatula.)
2) Reduce the speed of the mixer to low. Add the double cream, vanilla and zest. Raise the speed to high and mix briefly until fluffy (scrape down the bowl occasionally).
Store in the refrigerator until somewhat stiff before using. May be stored in the refrigerator for three days.