Directions

1. Preheat oven to 325 degrees. Brush madeleine pans well with 4 tablespoons softened butter; refrigerate 5 minutes.
2. In a small heavy-bottomed saucepan, melt remaining 1/2 cup butter over medium heat. Continue to cook until butter turns golden brown, being careful not to let the butter burn. Remove from heat and strain through a fine mesh sieve into a small bowl; set aside.
3. In the bowl of an electric mixer fitted with the whisk attachment, whip eggs with granulated and brown sugars until light and thickened, 3 to 5 minutes. Add flour, cocoa powder, baking powder, and salt, and stir on low speed until combined. Add vanilla and reserved browned butter; mix just until blended.
4. Insert a 1/2-inch plain tip into a pastry bag and fill with batter. Pipe mounds of batter into prepared pans until each mold is full, mounding batter in the center of the molds but not filling to the edge. Alternatively, you can use a teaspoon to fill the molds with batter. Bake until madeleines are firm and a little mound is puffed up in the middle, 10 to 12 minutes. Madeleines are best eaten the same day they are baked.