Put the butter in a small saucepan over a medium heat until it foams and turns a nutty brown colour – watch the pan carefully, as butter can burn quickly. Pour into a bowl and set aside to cool.
Beat the eggs, sugar and zest in a large bowl with an electric hand whisk for about 5 mins until thick and pale. Add the juice and mix to combine. Mix the flour and baking powder together, then sift over the batter mixture, a third at a time, and fold in.
Add a large spoonful of the batter to the butter, mixing together until smooth and combined. Pour this into the batter and gently fold together until fully combined. Press a sheet of cling film onto the surface of the mixture and chill overnight.
About 30 mins before you want to bake the madeleines, grease a 12-hole madeleine tin with a little extra butter and dust with a little flour. Heat oven to 200C/180C fan/gas 6 and put the tin in the freezer to chill for 30 mins. Remove the batter from the fridge and add about 1 heaped tbsp to each madeleine hole – be careful not to overfill as they will bake over the edge and won’t form the characteristic hump. Depending on the size of your tin, you will have some batter left – either bake in batches or put the rest back in the fridge and bake within three days.
Bake in the oven for 10-12 mins or until golden brown around the edges. Immediately turn out the madeleines onto a wire rack and leave to cool fully. Put the chocolate in a microwaveable bowl and heat in the microwave on High in 30 sec bursts until about three-quarters of the chocolate has melted. Remove from the microwave and stir vigorously until the remaining chocolate has melted (this is a simple method of tempering and just ensures the chocolate will set properly at room temperature). Dip each madeleine halfway into the melted chocolate, allowing the excess to drip off, then place on a sheet of baking parchment. As the chocolate starts to set, but while still a little tacky, sprinkle with some of the Blood orange dust (below), if using, and allow to cool.