Preheat oven to 350 deg. Butter 30 madeleine molds. Melt chocolate in top of double boiler set over hot water. Stir in butter. Mix together flour, baking powder and salt. Beat eggs in large bowl of electric mixer until light and lemon colored; add vanilla. Gradually add sugar, beating constantly at high speed 5 minutes. Reduce speed to medium and beat until mixture has quadrupled in volume, about 5 minutes. Gently fold in flour mixture, then chocolate. Place 1 tablespoon batter in center of each mold. Bake exactly 12 minutes. (Madeleines should be moist and fudgy in center.) Cool on rack. Dust with powdered sugar before serving. (Can be prepared 1 day ahead. Store in airtight container.)