Preheat the oven to 200 degrees celsius and grease a madeleine tray with butter.
Melt the chocolate and butter either over a pot of steaming water or in the microwave and stir until smooth. Set aside to cool.
In a small bowl, sift together the flour and baking powder.
In a mixer, whisk the eggs until frothy. Then gradually add the sugar and continue whisking until the mixture is pale yellow and has thickened. Turn off the mixer.
Add the cooled butter to the mixture and fold through the mixture gently with a spoon before carefully adding the flour. Mix until just combined.
Fill the moulds until ¾ full and bake in the oven for 11 to 13 minutes or until they are puffed and spring back when touched. Do not overbake.