Sift the all-purpose flour and sugar into a small bowl. Add the yeast, then ½ cup lukewarm milk and whisk until smooth. In a separate bowl, combine the buckwheat flour and salt. Add the egg yolk and remaining milk; whisk until smooth. Cover both bowls with plastic wrap and set aside at room temperature for 1-4 hours.
After resting, whisk together the yeast mixture and buckwheat mixture.
Whisk the egg white in a small bowl until soft peaks form, then transfer to the batter bowl. In the used egg white bowl, add heavy cream and whip until soft peaks form. Transfer to the batter bowl and gently fold in the egg whites and cream with the batter. Cover the bowl with plastic wrap and set aside at room temperature for one hour. You may allow the batter to rest longer (up to 24 hours), where the batter will develop more concentrated flavors and blinis will cook crisper.
Pour melted butter into a non-stick Swedish pancake pan (or small skillet) over medium-high heat. Drop ½ Tbsp of batter into the pan and spread to form the ideal size blini (2-inches is recommended). Cook until golden brown (about 10 seconds), then turn and cook until the other side is golden brown. Remove from the oil and place on a paper towel-lined plate to cool. Repeat through the remaining batter.
Top each blini with a dollop of crème fraiche, a small spoonful of caviar and additional garnishes as desired.