In a bowl, mix 200 grams of almond flour, 100 grams of powdered sugar, 1 teaspoon of vanilla extract, and 100 grams of softened unsalted butter to make the almond cream. Set aside.
Preheat the oven to 180°C (350°F).
Roll out 500 grams of puff pastry on a lightly floured surface into a large rectangle (about 40x30 cm). Cut the rectangle into 6 equal squares.
Place a generous spoonful of almond cream in the center of each square. Fold the corners of the square towards the center, sealing the almond cream inside. Gently press the edges to seal.
Place the filled croissants on a baking sheet lined with parchment paper. Brush the tops with a beaten egg for a glossy finish.
Sprinkle the croissants with sliced almonds and a little granulated sugar.
Bake for 20-25 minutes, or until the croissants are golden brown and puffed up.
Allow the almond croissants to cool slightly before serving.