1. Heat oven to 375 degrees F. Coat 24 indentations in madeleine pans (see Note) with nonstick cooking spray.
2. Into medium-size bowl, sift together flour, cocoa, cornstarch, baking powder and salt. In large bowl, with electric mixer, beat together butter and sugar until smooth and creamy, about 2 minutes. Beat in eggs, yolks, rum and vanilla 1 minute or until blended. On low speed, beat in flour mixture just until combined.
3. Spoon slightly rounded tablespoon batter into each indentation.
4. Bake in 375 degrees F oven 10 minutes or until toothpick inserted in centers comes out clean. Let madeleines cool in pans on rack 5 minutes. Remove madeleines from pan to rack; let cool. Dust with sugar before serving.