Melt the butter in a skillet over medium heat.
Toast the buns cut-side down until golden and crisp.
Combine crème fraîche and chives in a small bowl.
Spoon a generous layer of the crème fraîche mixture onto each top bun.
Add a large dollop of caviar to each bottom bun — no skimping!
Sprinkle with lemon zest.
Place the top bun on gently. Brush the tops lightly with the remaining melted butter and warm in a toaster oven for a couple minutes (optional).
Serve immediately with a chilled glass of bubbly or something crisp like mineral water with a squeeze of lemon or lime.