Cream together butter and sugar until fluffy, then stir in the eggs one at a time, beating well after each addition. Mix the flour with the baking powder and the salt and mix in the cocoa. Stir dry ingredients into the egg mixture. Cover and chill for about 1 hour in the refrigerator. Preheat the oven to 180°C (approximately 350°F) convection.
Grease the madeleine pan wells (each 5 cm/approximately 2 inches), dust with flour and fill the wells 2/3 full with the batter. Bake on a rack in the preheated oven for about 15 minutes.
Remove pan from oven and let cool slightly. Remove cookies from the wells and allow to cool on a wire rack before serving.