Take everything out of the fridge in good time so it reaches room temperature (this makes a huge difference to the lightness of the cakes) Preheat the oven to 200 degrees C.
Put all the ingredients for the buns, except the milk, into a food processor and blitz until smooth.
Pulse while pouring the milk into the funnel, to make a smooth dropping consistency.
Divide the mixture into a 12-bun tin lined with muffin papers and bake for 15-20 minutes until risen and golden on top (you want a peak if possible).
Let them cool for a while in the tin then move to a wire rack to cool completely.
Now for the topping. To make the marshmallow base you need to whisk the egg whites over heat until they're stiff and gleaming.
Fit a bowl over a saucepan of barely simmering water and put in all the icing ingredients except the vanilla and sprinkles. Whisk with an electric whisk for about 5 minutes.
When it has become thick and hold peaks like a meringue, take the bowl off the saucepan and whisk in the vanilla. Keep whisking while the mixture cools a little.
The icing mixture should be peaked. Spoon some over each bun, then swirl another spoonful on top to get the whipped look. The icing sets quickly so sprinkle the decorations over ASAP.