1. Make madeleines: Preheat oven to 400°F. Brush molds with some melted butter.
2. Whisk together cornmeal, flour, sugar, baking soda, salt, and pepper in a bowl. Whisk together egg, buttermilk, 1 tablespoon melted butter, and corn in another bowl, then add to dry ingredients, stirring until just combined.
3. Spoon 1 teaspoon batter into each mold and bake in middle of oven until madeleines are golden around edges and spring back when pressed lightly, 5 to 6 minutes. Turn madeleines out onto a rack to cool and make more in same manner.
4. Top madeleines: Put sour cream in pastry bag (if using) and pipe (or spoon) about 1/2 teaspoon onto each madeleine, then top with a rounded 1/4 teaspoon caviar.