Melt the butter in a saucepan. Beat the eggs and sugar for a few minutes until very light. Add the melted butter, flour, cocoa and salt. Mix until the dough is even.
Colour the dough with the food coloring. The amount you need will vary depending on the coloring you use.
Chill in the fridge for at least 2 hours. Grease the tins with butter or baking spray. Divide the dough between the moulds.
Bake the madeleines for 8-10 minutes at 200˚C/400˚F (conventional oven).
As soon as the madeleines come out of the oven, turn them over a wire rack; if you wait, they will be much harder to remove from the pan. Leave to cool completely.
Put the white chocolate in a disposable icing bag and hang it in a glass of warm water. Wait until the chocolate has completely melted. Then cut off the tip of the piping bag and pipe the contents over the madeleines. Decorate some (or all) with sprinkles.
Tips
No piping bag? Melt the chocolate in a small bowl and drizzle over the madeleines with a spoon.