Preheat oven to 350°F. Brush bread with melted butter and cut out 40 moons. Arrange, buttered sides up, on a large baking sheet and bake in middle of oven until pale golden, about 10 minutes. Cool completely. Serve toasts topped with sour cream and caviar.
Other Notes
Toasts may be made 1 day ahead and kept in an airtight container at room temperature.