In a medium bowl, whisk together buckwheat flour, baking powder, dry mustard powder, salt, and freshly ground pepper.
Gradually add skim milk and stir until just combined.
Fold in the snipped chives and shredded Cheddar cheese.
In a separate clean bowl, beat egg whites until stiff peaks form.
Gently fold the beaten egg whites into the batter, preserving as much airiness as possible.
Heat a nonstick skillet or griddle over medium heat and lightly grease it.
Drop small spoonfuls of batter (about 2 tablespoons each) onto the skillet to form blini.
Cook for 2–3 minutes or until bubbles form on the surface and edges look set; flip and cook another 1–2 minutes until golden brown and cooked through.
Transfer blini to a plate and keep warm; repeat with remaining batter.
Top each warm blini with a slice of smoked salmon and a sprinkle of additional chives if desired.
Serve immediately as a delicious appetizer or light meal.