Heat the milk with the vanilla in a saucepan.
Meanwhile, in a mixing bowl, whisk the egg yolk with the sugar, then add the cornstarch. Mix well.
Pour the hot milk over the mixture while stirring, then pour everything back into the saucepan.
Cook over medium heat, stirring constantly, until the pastry cream thickens.
Transfer the pastry cream to a container, cover directly with plastic wrap to prevent a skin from forming, and let cool.

In a mixing bowl, combine the butter and sugar until smooth.
Add the egg and mix.
Add the almond flour and mix again.

Fold in the cooled pastry cream and mix until combined.

Place one sheet of puff pastry on a baking sheet lined with parchment paper.
Spread the filling over the pastry, leaving a 3/4-inch (2 cm) border around the edges.
Add the fève (small charm or bean).
Brush the edge of the pastry with a little water.
Place the second sheet of puff pastry on top and gently press the edges to seal.
Brush the top of the galette with a mixture of egg yolk, heavy cream, milk, salt, and sugar.
Use the tip of a knife to draw designs on the pastry.

Chill in the refrigerator for 30 minutes.

Brush the top of the galette with the egg wash again, then bake for about 35 minutes, depending on your oven.