Whisk together the flours, salt and sugar in a mixing bowl. Warm the milk to body temperature in a saucepan, then stir in the yeast until it dissolves. Whisk in the sour cream, and the egg yolk.
Pour the liquid ingredients into the flour and whisk thoroughly. Cover the bowl with a tea towel and put it in a draught-free place to rise for an hour (it might take a little longer than this on Christmas morning, so feel free to get your bird prepped, or stuffing made). The batter should almost double in size.
After an hour, beat the egg whites to stiff peaks, and then fold them into the frothy mixture. Cover with the tea towel again and leave for another hour. The mixture should be very light and full of bubbles –almost like a foam.
Once the batter has risen, warm half a tablespoon of the butter in a frying pan. Without stirring the mixture (you want to retain the lightness), drop teaspoons of the batter into the pan. When the top of a blini is covered with bubbles, flip it over. Cook the blinis in batches until all the batter is used up, adding more butter when needed.
Serve each blini warm, with a twist of salmon, a dollop of sour cream, a sprig of dill, and a squeeze of lemon. They can be warmed through in the oven, but are best fresh, if you can serve them straight away.