Caviar and Salmon Blini Tortes

INGREDIENTS

• 1/8 teaspoon baking soda
• 1/4 lb thinly sliced smoked salmon at room temperature
• 1/2 stick unsalted butter, melted and cooled, divided
• 2 tablespoons sour cream
• 1/8 teaspoon salt
• 1 tablespoon finely chopped chives
• 2 ounces trout caviar, such as rainbow- trout caviar, or salmon roe
• 1/4 cup whole milk
• 1 large egg, separated
• 1 hard-boiled large egg, finely chopped
• 1 teaspoon sugar
• 3 tablespoons all-purpose flour
• Garnish: sour cream; chopped chives
• 2 tablespoons buckwheat flour