King Cake with Almond Cream (Galette des Rois)

Preheat the oven to 200°C (390°F).
In a bowl, mix together the powdered sugar, ground almonds, and cornstarch.
In another bowl, beat the butter with this dry mixture until smooth.
Gradually add the eggs, then finish by mixing in the almond milk. Set aside.
Mix the whole egg, egg yolk, and water together. Keep chilled.
In a saucepan, combine the sugar and water, bring to a boil, then set aside.
Cut two circles of puff pastry, 18–22 cm (7–9 inches) in diameter.
Place a silicone baking mat or parchment paper on a baking sheet. Lay down one pastry circle and spread the almond cream evenly on top (don’t reach the edges).
Place the second pastry circle on top and seal the edges well. Decorate the edges by lightly notching them with a small knife.
Brush the top lightly with the egg wash. Using a small knife, score a decorative pattern on the surface.
Bake for 40 minutes.
When the cake comes out of the oven, brush a thin layer of syrup over the top. Let cool before serving.