Whisk the first 5 ingredients (through salt) in a medium bowl.
Place the milk and butter in small saucepan. Stir over low heat until butter melts and thermometer registers 110°F (if mixture gets too warm, cool until temperature returns to 110°F). Pour warm milk mixture into flour mixture and whisk until smooth. Cover with plastic wrap and let stand in warm draft-free area until doubled in volume, 1 to 1½ hours.
Whisk buckwheat batter to deflate; then, whisk in eggs. (Do ahead tip: Can be prepared to this point 1 day ahead. Transfer to large bowl. Cover; chill (may increase in volume; re-whisk before using.)
Preheat oven to 200°F. Heat a griddle or large skillet over medium heat. Brush lightly with melted butter. Working in batches, pour a large tablespoonful of batter for each blini onto griddle, spacing apart. Cook until bubbles form on top and begin to pop, about 1½ minutes. Turn blinis and cook until golden brown on bottom, about 1½ minutes. Transfer to baking sheet. Tent with foil and place in oven to keep warm while cooking remaining blinis. (Do ahead tip: Blinis can be made 1 day ahead. Cool, cover, and chill. Brush blinis lightly with melted butter and rewarm on baking sheet in 350°F oven 5 minutes.)
ASSEMBLY
Arrange warm blinis on platter.
Top with piece of salmon and then a small dollop of crème fraîche. Garnish with a small dill sprig and serve
Notes
If you'd like a clean look to the pancake, you may wish to use a scallop-edged cutter to cut into the little pancakes.