French novelist and notorious shut-in Marcel Proust was passionate about the plump little cakelike cookies called madeleines. Perhaps the aroma of this chocolate version baking would have drawn him out of his room.
1 ounce bittersweet (not unsweetened) chocolate or confectioners' sugar, for decoration
Preheat oven to 400°F. Coat a madeleine pan with cooking spray. Dust with flour, shaking out excess.
Place whole egg and egg white in a mixing bowl and set the bowl in a larger pan of hot water to warm while you are preparing the remaining ingredients. Stir the eggs occasionally.
Sift cocoa, flour, baking powder, baking soda and salt into a mixing bowl; set aside. Combine buttermilk and oil in a small bowl; set aside.
Take the egg bowl off the water, add sugar and beat with an electric mixer on high speed until the mixture is thickened and pale, about 5 minutes. (The beaters should leave a ribbon trail when lifted.) Blend in vanilla and coffee (or orange zest). Alternately fold the dry ingredients and the buttermilk mixture into the egg mixture with a rubber spatula, making 3 additions of dry ingredients and 2 additions of liquid.
Drop the batter by tablespoonfuls into the prepared pan, filling each depression about three-quarters full; you will use about half of the batter. Bake until tops of the madeleines spring back when touched lightly, 12 to 15 minutes. Immediately loosen the cakes from the pan and invert onto a rack to cool. Clean and prepare the pan as above and repeat with the remaining batter.
If decorating with chocolate, melt it in a small bowl set over a pan of barely simmering water. Drizzle over the scalloped side of the madeleines. Alternatively, dust the madeleines with confectioners' sugar.
Make Ahead Tip: The madeleines are best eaten the day they are baked, but they can be wrapped and frozen for up to 1 month.