Sift the buckwheat flour, plain flour and baking powder together into a bowl. Mix the yeast with the milk and egg yolk.
Whisk the yeast mixture into the dry ingredients to make a smooth batter. Stir in 1 tsp of melted butter.
Whisk the egg white to stiff peaks in a clean bowl using an electric whisk, then gently fold this into the batter, keeping as much air in the mixture as possible.
Gently melt 125g of butter in a pan, then pour off the clear yellow liquid into a jug (discard the white bit left behind in the pan). Heat the clarified butter in a frying pan and drop in dessertspoonfuls of the batter. Cook until the surface starts to bubble, then flip and cook the other side. Serve with soured cream and smoked salmon on top.