Preheat the oven to 180°C.

In a mixing bowl, combine the softened butter and sugar until you have a smooth paste. 
Crack the eggs (2 eggs for a galette des rois) and add a pinch of fleur de sel. Mix well.
Add the ground almonds and mix again until you have a smooth cream.
On a baking sheet or rack lined with parchment paper, roll out the first sheet of puff pastry and prick it with a fork.
Spread the almond cream, leaving a 3 cm border, then hide your charm. Be careful not to put anything that could melt during baking!
Moisten the edges of the dough with a little water, then place the second sheet of puff pastry on top. Remove any air bubbles and seal the edges well.
Remove the baking paper from the second sheet of puff pastry, then place a dish or large round plate (ours is 10 inches in diameter) on top to even out the edges of the galette. Make small cuts with a knife or fork around the edge of the galette to give it a nice shape.
Separate the yolk and white of the last egg. In a bowl, mix the yolk with a little water, then brush the galette with a pastry brush.
Using the tip of a knife, carefully draw the desired pattern on the galette. Prick the galette, then bake it for 30 minutes at 10°C.
Once the galette is golden brown and puffed up, remove it from the oven.