These dainty treats are filled with homemade orange curd. You’ll make more curd than you need, so keep the remainder to have on toast another time.
Heat the oven to 200°C/180°C fan/400°F/Gas 6.
Make the sponges. Whisk together the eggs and sugar in the bowl of a stand mixer for 5 minutes, or until the mixture is thick enough to form a ribbon trail when you lift the whisk.
In a separate bowl, sift together the flour and baking powder, 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 the orange zest and mix until completely combined. Leave to rest for 20 minutes.
Meanwhile, make the orange curd. Mix all the ingredients apart from the butter in a small pan over a medium–low heat. Stir continuously for about 10 minutes (don’t rush or the mixture might scramble), until thick enough to coat the back of a spoon.
Remove the pan from the heat, stir in the diced butter to melt, then transfer the mixture to a shallow dish. Cover the surface with cling film, then refrigerate.
Spoon the sponge mixture evenly into the prepared madeleine tray and bake for about 10 minutes, until the sponges are risen and just golden brown at the edges. Cool in the tray for 3 minutes, then transfer to a wire rack to cool completely.
Place 6 tablespoons of orange curd into 1 piping bag and snip the end (keep the remaining curd in the fridge, for use in the week). Push a skewer into each madeleine from the square end into the thickest part to make a hole. Push the end of the piping bag into the centre of each madeleine and pipe a little curd into each one.
To make the decoration, place the dark chocolate in a small glass bowl and microwave in short 20-second bursts for about 2 minutes, until melted.
Repeat for the white chocolate, microwaving in short, 20-second bursts for about 1 minute, until melted, then place in a small piping bag.
Dip the square end of each filled madeleine in the dark chocolate and place on a wire rack. Snip the end of the white-chocolate piping bag and pipe the white chocolate across the dark in 2 or 3 lines. Use a cocktail stick to drag the lines of white first one way, then the other, to make a feather pattern.
Sprinkle the edges of each madeleine with chopped hazelnuts and refrigerate for 5 minutes to set before serving.