In a medium bowl, whisk the buckwheat flour, baking powder and dry mustard. Whisk in the milk until smooth. Add the 1 1/2 tablespoons of chives and the cheddar and season the batter with salt and pepper.
In another bowl, beat the egg whites with a pinch of salt until firm peaks form. Whisk one-fourth of the whites into the batter. Fold in the remaining whites.
Heat a griddle over moderate heat until very hot and spray with vegetable oil spray. Spoon 1-tablespoon mounds of batter onto the griddle and cook until bubbles form around the edges of the blini, about 2 minutes. Flip and cook the blini for about 30 seconds longer.
Transfer the blini to a platter and top with the smoked salmon and a dollop of sour cream. Garnish the blini with chives and serve right away.