Brush a madeleine baking pan with the melted butter.

Put remaining butter and the chocolate in the top of a double boiler or in a heatproof bowl set over a pan of simmering water (be sure the water doesn’t touch the bowl). Heat until chocolate and butter are almost melted, 3-5 minutes. Remove from heat and stir together until smooth.

Sift powdered sugar, flour, cocoa and salt into a bowl using a mesh strainer.

Beat egg whites in a large bowl with an electric mixer or by hand until very frothy, about 2 minutes. Stir in flour mixture until combined. Add chocolate mixture and mix with a spatula until well blended.

Portion batter into the pan, filllig each cup with about 1 heaping tablespoon of batter. Chill in the refrigerator one hour. If your pan makes only 12 at a time, chill remaining batter in a bowl and bake in batches.

Preheat oven to 375 degrees.

Bake about 12 minutes, or until madeleines are springy to touch. Cool in pan 1 minute, then unmold madeleines onto a rack. Serve warm or at room temperature.