Begin by making a double espresso or an extra strong coffee in a cafetiere and leave it to cool in a fridge while you prepare the Madeleine mixture.
Slowly melt the butter in a pan, but don't let it bubble. Then leave it to cool to room temperature.
Preheat the oven to 200˚C/Gas 6. Lightly grease the Madeleine pan with a little extra softened butter.
In a large bowl, whisk the eggs and sugar with an electric hand mixer for 3-4 minutes, until pale and frothy.
Sift in the flour and baking powder, then add the cold coffee and melted butter and lightly whisk into the egg mixture, until just combined. Leave to stand for 15 minutes.
Add a teaspoon of the mixture to the prepared baking tin then bake for 8-10 minutes until well risen, golden brown and springy to the touch. Leave in the tin to cool for 5 minutes, then turn out onto a wire rack.