For the hazelnut frangipane:

1. Heat milk with vanilla in a medium saucepan, and stop when it starts bubbling.
2. In a medium-size bowl, beat yolks with ¼ cup (40g) sugar until pale and fluffy. Whisk in the flour combined with the cornstarch, then pour in the hot milk, while whisking constantly.
3. Put the vanilla milk mixture back in the saucepan and simmer for a few minutes until the mixture thickens, whisking constantly. Transfer this crème pâtissière into a medium-size bowl, and cover with cling film, creating contact with it to prevent from drying on top. Let chill in the refrigerator.
4. Meanwhile, mix the softened butter and the remaining ½ cup (100g) sugar in a medium-sized bowl until creamy. Add in the eggs, one at a time, whisking between each addition. Pour the rum and the hazelnut meal, and stir well.
5. Combine the crème pâtissière and hazelnut cream, creating a hazelnut frangipane.
6. Add grated dark chocolate, combine, and transfer the frangipane into a piping bag. Place in the refrigerator until ready to use.

For assembling the galette des rois:

1. Preheat the oven to 350°F (180°C) and line a large baking sheet with parchment paper.
2. Place the smaller disk of puff pastry* onto the prepared sheet, then pipe the hazelnut frangipane into circles, starting from the center, leaving about 0.5 inch (1,5 cm) margin all over. Gently brush a little water on the margin. Don’t forget to hide the fève.
3. Cover with the second disk, press with your thumb to seal the edges, and make small cuts with a knife all around.
4. Whisk the yolk with maple syrup. Brush the galette des rois, and draw some patterns of your choice on top, using a small knife (just draw, do not cut through!). Bake for about 30-40 minutes or until the galette is golden-brown. Serve warm or lukewarm, with a glass of French apple cider, and enjoy!