Preheat oven to 375°. In a medium bowl, whisk together flour, baking powder, and salt. In another bowl, vigorously whisk egg, egg white, and granulated sugar until blended. Add flour mixture and whisk until just combined. Whisk in 6 tbsp. butter and the corn kernels.
Generously butter a 12-compartment madeleine pan or mini-muffin pan (2-tbsp. size) with remaining 1/2 tbsp. butter. Spoon batter evenly into cups (they'll be full).
Bake madeleines until they're light golden and spring back when lightly pressed in center, 12 to 15 minutes. Loosen from pan with a knife and lift out to a plate.
Serve madeleines warm or at room temperature, dusted with powdered sugar.
Note: Nutritional analysis is per madeleine.