Melt the butter either in a small pan or in the microwave in a small dish. Set aside to cool while you prepare the rest.
Whisk together the sugar and egg until the mixture becomes paler and a little thick, a good few minutes.
Sift in the cocoa powder, flour, baking powder and salt. Fold them in (with a spatula is good) so they are well mixed but without over-mixing. Fold in the melted butter and once again, mix but don't over-mix.
Cover the bowl and refrigerate for about an hour. Alternatively you can chill a few minutes in the freezer.
Once you are almost ready to bake, preheat the oven to 350F/175C. Brush the madeleine pan lightly with melted butter.
Take the mixture from the fridge and gently loosen it from the bowl but don't mix out any of the air. Spoon tablespoonfuls of the mixture into each mould of the pan, without spreading it out at all.
Put the pan in the oven once it has reached temperature and bake for approximately 12 minutes until they are lightly browned and with a little hump in the middle.
Allow to cool a minute before turning them (invert) onto a cooling rack, with the pattern side now up.
Once they are largely cool, melt the semisweet and white chocolate separately. Put the madeleines close together on the cooling rack over a surface you don't mind getting a little messy and drizzle over the dark and white chocolate. Leave to set. Best enjoyed the day they are made so they are slightly crisp though they will keep a day or two - they just become slightly fudgy.
If you want to add the dusting of pink, as I have on some, simply crush some freeze dried strawberries and sprinkle over while the chocolate drizzle is still wet. You'll need around 1tbsp if you do all.