Begin by sifting the buckwheat flour, plain flour and salt together into a large roomy bowl and then sprinkle in the yeast. Place the crème fraîche and milk in a small saucepan and warm it gently – it must only be slightly warm, as too much heat will kill the yeast.
Next add the egg yolks to the milk, mix them in with a whisk and after that pour the whole lot into the flour mixture. Whisk everything until you have a thick batter, then cover the bowl with a clean tea cloth and leave it in a warm place for about 1 hour – this can simply be a matter of placing the bowl in another larger bowl filled with warm water.
After 1 hour the batter will be spongy and bubbly, now you whisk up the egg whites until they form stiff peaks and gently fold them into the batter. Cover with the cloth again and leave as before for another hour.
When you're ready to make the blinis, begin by melting the butter in a heavy-based frying pan, then tip the melted butter out into a cup and use it with the help of a tightly rolled wodge of kitchen paper to brush the pan all over as you make each blini.
To do this, keep the pan on a medium heat and add 1½ tablespoons of batter – (1 tablespoon goes in first, then another ½ tablespoon on top) – it won't spread out much and the underneath sets as soon as it touches the pan. This amount should give you a blini about 4 inches (10 cm) in diameter. Don't worry at this stage if it looks too thick, it isn't, it's just light and puffy.
After 40 seconds, no longer, flip the blini over and give it just 30 seconds on the other side. Transfer it to a wire cooling-rack and repeat, brushing the pan with butter each time. This mixture should give you 18-20 blinis.
When all the blinis are made and have cooled, wrap them in foil parcels, with 6 laid out flat in each one.
To serve, pre-heat the oven to gas mark 1, 275°F (140°C) and place the foil parcels on a high shelf for 10 minutes.
Serve the blinis on warm plates, giving each person 2 to start with, and top with slices of smoked salmon, about 2 oz (50 g) per person, add a tablespoon of very cold crème fraîche on the side of the plate and garnish with sprigs of fresh dill.