For the Blinis
Stir yeast, 2 Tbsp flour and 1 cup milk to combine, cover and let rest at room temperature for 20 minutes.
Stir in remaining 1/2 cup flour, buckwheat flour, egg yolks and salt until evenly blended.
Whip whites to soft peaks and set aside.
Whip cream to soft peaks and fold into flour mixture. Fold in whipped whites in 2 additions and immediately cook blinis.
Lightly grease a griddle over medium heat.
Drop batter by teaspoonfuls onto griddle and cook for 2 minutes.
Gently flip blinis over and cook 1 minute more. Remove to a plate and repeat with remaining batter. Wrap and store blinis at room temperature until ready to serve or freeze.
For the Smoked Trout Mousse
In a food processor, pulse smoked trout and cream cheese, scraping bowl often. Add cream and lemon juice and pulse until smooth.
Scrape mousse into a bowl and stir in chives, dill and pepper. Chill until ready to serve.
Chill until ready to serve.
To assemble, pipe mousse onto blinis and garnish with a small dill sprig.