These are simplicity itself to make, but so light and delicate – the perfect accompaniment to a cup of coffee or to serve after dinner as petits fours.
Heat the oven to 220°C/200°C fan/425°F/Gas 7. Melt the butter in a small bowl in the microwave for 40 seconds to melt. Pour 5g of butter into a tiny bowl and use this to brush the madeleine tray. Dust with flour, then chill the tray in the freezer. Set aside the remaining 90g of butter.
Place the eggs and coffee liqueur in the bowl of a stand mixer fitted with the whisk. Whisk on high speed for 2 minutes, until the mixture is thick and foamy. Add the sugar, then whisk for another 5 minutes, or until the mixture is thick enough to form a ribbon trail when you lift the whisk.
In a bowl, sift together the flour, baking powder and salt, then sift a quarter of it over the egg mixture and gently fold in to combine. Gently fold in the remaining flour, then the cooled, melted butter and mix until completely combined. Spoon the batter evenly into the madeleine tray and freeze for 5 minutes.
Bake the madeleines for 8 minutes, until risen and just golden brown at the edges. Cool in the tray for 3 minutes, then transfer to a wire rack to cool completely.
Meanwhile, make the decoration. Place the chocolate in a small glass bowl and microwave in short 20-second bursts for about 2 minutes, until melted.
Once the sponges are cool, dip the scalloped edge of each madeleine into the melted chocolate. Return to the cooling rack and sprinkle over the crushed coffee beans. Place in the fridge to set, then spray the chocolate with gold lustre before serving.