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 sugars, molasses and egg until the mixture becomes paler and a little thick, a good few minutes.
In a separate bowl, whisk together the flour, baking powder, spices and salt so they are well blended. Add them a little at a time to the egg mixture (with a spatula is good, or else continue to use mixer). You want it well combined but not over-mixed. Fold in the melted butter and once again, mix but don't over-mix.
Cover the bowl and refrigerate for about an hour. Alternatively, leave overnight to let the flavors mix more then take out a few minutes before you will use.
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, gently dust with powdered sugar (optional) and enjoy. Best enjoyed when right out the oven, gently warm. They will keep a day or two but will lose their crisp edge.