For the madeleine batter: Firstly, place the butter in a small saucepan over a medium-low heat and cook until melted. Then remove from the heat and set aside. Then in a large bowl, whisk together the eggs and mint extract until well combined. Then whisk in the sugar, until well combined and lighter in colour. Then whisk in the golden syrup.
In a separate small bowl, whisk together the flour, baking powder and cocoa powder until combined. Then whisk into the egg mixture until smooth. Finally, whisk in the lukewarm melted butter and then the chocolate chips. Place the mixture into a piping bag, and place into the fridge to chill for one hour. (If you don't have a piping bag, keep the mixture in the bowl and place a sheet of clingfilm directly on the surface of the batter).
While the batter is chilling, using a pastry brush, brush the madeleine pan with the butter, ensuring you grease all the crevices. And preheat the oven to 210C/190C Fan/Gas Mark 8.
Pipe (or spoon) the chilled batter into your prepared madeleine tin, dividing evenly between all the shells. (You don't need to spread the mixture out, as it will spread when in the oven.) Bake for 12-14 minutes until a hump has formed, then remove from the oven and immediately turn the madeleines out onto a cooling rack.
To decorate: While the madeleines are cooling, place the candy canes into a sealed sandwich bag and using a rolling pin, crush into small chunks. Then melt the dark chocolate in either a microwave in 30-second increments, or over a bain-marie. Dip the madeleines, shell side down, into the dark chocolate and place onto baking paper to set, sprinkling the crushed candy canes on top. Repeat the process for all of the madeleines.