Bring the heavy whipping cream to a boil. Pour it on the chocolate, wait a few seconds then mix well until the mixture is homogeneous.
Beat the softened butter and sugar together.
Beat in the egg. Mix.
Then, add the almond flour and mix well.
Add the melted chocolate to the almond preparation and mix well. If this preparation is too thin (because it is still hot), leave to cool in the refrigerator for 10-15 minutes.
Roll out one sheet of puff pastry. Spread the chocolate-almond filling over the center of the dough, leaving a 1 inch (1.2 cm) exposed border. If you wish, place a little porcelain figurine "fève" in the filling.
Brush the edges of the pastry with water and cover with another circle of dough on top and press down to seal the edges.
If you want, with a knife or just with your fingers, decorate the edges.
Brush with beaten yolk. Place in the refrigerator for at least 1 hour.
Preheat oven to 390F (190C).
Brush with yolk one more time. Using a knife, create a design on top of your galette. Poke 3-5 holes, to allow steam to escape while baking.
Bake for 30 minutes or until the pastry is golden brown.